Limited vision

For many scholars of the early modern world, studies of natural history were unthinkable without the first-hand, arduous experience of personal travel. Like above-ground journeys, excursions underground followed predetermined itineraries and were associated with visual expectations, encounters and illusions. Since Plato's analogy of the cave, subterranean sites, with their inherent liminality, have been understood as the prototype of an epistemic laboratory where the limitations of human perception are examined in an experimental setting.Footnote ³¹ According to contemporary accounts, the multi-sensory experience of descending into heavy and humid darkness and encountering objects of uncertain origin not only overwhelmed many naturalists, but also undermined their trust both in their own observations and in the descriptions of previous visitors.

The Protogaea, a posthumously printed treatise on the history of the Earth by the prominent German polymath Gottfried W. Leibniz (1646–1716), includes probably the best-known early modern map of a cave.Footnote ³² Based on a survey published fifty years earlier by Hermann von der Hardt (1660–1746) and modified most likely according to Leibniz's directives – or even by Leibniz himself – the unsigned map is a visual representation of the detailed report of his subterranean sightseeing trip to Baumannshöhle cave (Harz mountains). In this report, Leibniz addresses the curiosities, such as mineral formations and fossils, that he encountered along the way and their distorted perception by many visitors:

Leibniz recognized caves as mutable spaces where the narrowing and delimitation of the visitor's field of vision generated an illusionary world shaped by the viewer's prior experience and imaginative power. Although he personally distinguished between in-depth observation and imagination, the map as revised by Leibniz unites both domains. While he took over some of the anthropomorphic depiction of stalactites and other curiosities from the previous survey, Leibniz also added highly detailed depictions of his own findings, such as petrified bones, which he inserted around the part adopted from von der Hardt's original map. His critical examination of sensory perception and the mind's reflection on what the senses perceive exemplifies the tension in which most early modern cave maps are embedded. As media of translation, cave maps engage with the dual function of the subterranean realm as a space of empiricism and imagination. The frequent reuse and adaptation of existing visualizations complied with the user's expectations of how the space should look, and satisfied their habits of reception.

The three-dimensional topography of caves, and the impossibility of illuminating them, mean that many areas (e.g. ceilings, slopes, deep pits or vast chambers) remain partly invisible. This in turn means that ‘translating’ subterranean landscapes into map format may have been even more demanding for surveyors than were above-ground topographic maps.Footnote ³⁴ In narrow passages or canyons, the field of view is blinkered and does not extend beyond the next turn. Similarly, the low intensity of the available light sources had a considerable effect on the observation of large-scale sections. For example, the English naturalist John Beaumont (c.1650–1731) reported on his 1681 visit to Lamb Leer Cavern (Somerset) in the Philosophical Collections that ‘by the light of our candles we could not fully discern the roof, floor, nor sides’ of the cave.Footnote ³⁵ Lowered by miners fourteen fathoms on a rope, Beaumont did not hold back from giving details of the dimensions of a vast chamber even though he himself could only see part of it.

Travellers and their guides chose from the broad variety of illuminants according to different needs. While wax candles were reserved for prestigious public and private rooms due to the scarcity of raw material, the ‘dull light’ of miners’ lamps and smoky pinewood spills could only illuminate a narrowly confined space.Footnote ³⁶ Torchlight – which is brighter, but produces a lot of smoke – met the visual expectations of an experience-oriented audience.Footnote ³⁷ Contrasting shadows transformed the ‘dark night of the cave’ into a ‘starry firmament … in which the stalactites shone like stars’.Footnote ³⁸ However, this did not satisfy the demands of the surveyors.

Maps display no evidence of these adversities, nor of the other visual limitations naturalists faced while crawling, descending or climbing in total darkness. The scholars’ awareness of enormous areas in the Earth's underground, which cannot be illuminated by torches or penetrated by human perception, gave rise to specific strategies for dealing with the invisible and establishing credibility. Rather than employing ‘cartographic silence’ – a term coined by John Brian Harley to designate spaces intentionally left blank in order to denote the unknown – cave maps dissected the Earth's subsurface by choosing a very detailed form of presentation and/or providing imaginative illustrations of hidden areas.Footnote ³⁹ Like artists, mapmakers were urged to ‘complete similarities that nature had only outlined’.Footnote ⁴⁰ But maps, which penetrated beyond the surface of the Earth and incorporated experiences gathered in exploration, were more reliable than paintings. Powerful tools of persuasion, ground plans and longitudinal sections lifted off the top or the side of a mountain and illuminated hidden geographies.

A comparison of two sketches of Kungur Ice Cave (Ural mountains), based on surveys by the Russian cartographer and polymath Semyon Remezov (1642–1720) and the German Johann Berckhan (1709–51), the painter attached to the second Kamchatka expedition, reveals these epistemic strategies. The first map (Figure 2A) was made in 1703 by order of Tsar Peter the Great and was subsequently copied by the Swedish military officer and cartographer Philipp von Strahlenberg (1676–1747), who printed the survey in his widely translated treatise on the Russian Empire.Footnote ⁴¹ The map guides visitors along an underground track past natural, unnatural and supernatural curiosities such as ice figures, an icon of Saint Nicholas and a fertile place where plants grow in darkness. Rather than leave invisible or unknown areas blank, the draftsman added fictional passages and chambers, emphasizing the cave's labyrinthine structure. Although this map became quite popular in eighteenth-century Europe and was reprinted in different styles and with variations in the objects depicted, the outlines of the galleries remained unchanged.

Figure 2.

(A) Ground plan of Kungur Ice Cave (Ural mountains), surveyed by Semjon Remesov in 1703 and published in a slightly modified version in Alexandre Deleyre (ed.), Histoire générale des voyages, vol. 18, Paris: Rozet, 1768, p. 105 (Tab. VIII). (B) Ground plan of the same cave, surveyed by Johann Berckhan in 1733 (Russian State Archive of Ancient Acts, F. 199, Opt. 2, Portfolio 430, D. 20, p. 1). As with other early modern cave maps, the authors used map symbols that mimicked the shape of natural specimens. Their understanding required no prior knowledge as they were easy to understand intuitively. Both maps even included formal legends or entire paragraphs that explained abbreviations and clarified the relationship between the signifier and signified. In short, the accessible sign systems were intended for a wider audience.

On the contrary, Berckhan's unpublished map (Figure 2B), drawn during a 1733 venture together with the German botanist Johann Gmelin (1709–55) and the historian Gerhard Müller (1705–83), uses another strategy to prove its credibility to potential users.Footnote ⁴² Although, as the survey indicates, direction was measured by a magnetic compass and length was counted in steps, the draftsman was aware of the insufficiency of his empirical observations underground and of the tools used. Accordingly, he uses shading and transitions between grey and black to emphasize the uncertainty of sight and of the exact position of the outlines of the galleries. However, he precisely marks the track that he and his companions took during their venture and explains the curiosities and religious monuments he encountered in more detail in the map legend. Even though this sketch also contains imaginary sections intended to reinforce the impressive scale of the cave, the depiction of the paths actually taken conveyed confidence in the validity of the survey. Overall, the phenomenon of ‘cartographic silence’ – in other words, the acknowledgement of empirical limits – is rarely, if ever, found in early modern cave maps.

Shifting boundaries underground

Caves are polyvalent places, as we can see by their diversity of use: as burial places, religious sites, dwellings, refuges, food stores, repositories for natural resources, features (in the form of grottos) in landscape gardens and popular destinations on the Grand Tour. This polyvalence is also evident in the research accounts of scholars of the early modern world. Alchemists and naturalists assigned special significance to caves and mines as access points to fertile processes such as mineral growth and the genesis of ores.Footnote ⁴³ Likewise, scholars experienced caves as ‘whole museums of stone’ that had apparently fallen into disarray, blurring the boundaries between the categories of natural history acknowledged in the scientific discourse of the time and making it difficult to identify the curiosities they encountered.Footnote ⁴⁴ Furthermore, the presence of famous caves in mining areas, such as the Harz mountains, upper Hungary and the Yorkshire Dales, meant that there was little differentiation of knowledge related to diverse underground spaces, with cave and mine studies arising and developing in parallel.

Canonizing the growing curiosity about underground sights, the guidebooks to subterranean destinations that appeared in eighteenth-century Europe were not limited to natural caves but also included tombs and catacombs; the ancient ruins of baths, cisterns and temples; and garden grottos and pits.Footnote ⁴⁵ Travellers distinguished less clearly than we might expect between natural and artificial cavities, as well as between the different kinds of objects discovered there.Footnote ⁴⁶ The ontological hybridity of these boundary spaces can be seen, for instance, in scholarly hypotheses about cave formation. Drawing on their visits to underground quarries and catacombs in Rome, Syracuse and Paris, a number of naturalists concluded that particular natural caves were also the result of human excavation. The majority of hypotheses in which a given cave was assumed to be artificial were based on the observation of individual objects, and did not in themselves constitute an overarching theory. They mostly concerned naturally formed caves that had been expanded by humans; for example, for use as sites of worship.Footnote ⁴⁷ For instance, the French naturalist Georges-Louis de Buffon (1707–88) argued that the caves of Arcy-sur-Cure (Bourgogne) were an underground quarry. Not long afterwards, the English topographer and antiquarian Samuel Rudder (c.1726–1801) maintained that Pen Park Hole near Bristol was a lead mine, no doubt on the basis that evidence of mining activity can be seen in parts of the cave.Footnote ⁴⁸

Referring to and incorporating scholarly assumptions about the origins of particular caves in their own work, mapmakers developed specific strategies to deal with the hybridity and shifting boundaries of underground spaces. This is particularly evident in the case of cavities of uncertain origin, where the lack of clear information meant that maps had to convey an especially compelling narrative. The first example I would like to mention in this respect is a map made by the French military officer Mathieu Dumas (1753–1837) of the Labyrinth Cave (Crete): a system of underground galleries, probably used as an ancient quarry (Figure 3A).Footnote ⁴⁹ The Labyrinth Cave was well known among travellers and was assumed to be the ruins of the mythological labyrinth of the Minotaur. Dumas investigated the cave in 1783 during a mission by order of the French king to gather information on the island's government and topography. Dumas's secret report to the sovereign, which included a map of Labyrinth Cave, was finally published in 1839.Footnote ⁵⁰ In the scholarly debate over the hybrid origin of the cave, Dumas excluded its use as an ancient quarry, since other sites were more favourable and the labyrinth had only one entrance. Rather, he argued that ‘some parts of this cave are a whim of nature, and existed before the hand of man added anything to it’.Footnote ⁵¹ Dumas's ground plan, surveyed using compass and pacing, serves as a convincing argument for this hypothesis, making visual the on-site observations described in more detail in his report.Footnote ⁵² The map shows no traces of quarry work and the outlines of the galleries largely resemble a natural cavity. However, the addition of architecture-like straight and rectangular elements is intended to indicate that the cave had been artificially enlarged, but only to a small extent. According to Dumas, his ‘exact copy’ of this hybrid space favoured its natural origin.Footnote ⁵³

Figure 3.

(A) Mathieu Dumas's 1783 ground plan of Labyrinth Cave (Crete) (Archives départementales des Yvelines, Collection graphique, FR/FR-AD078/A1604). (B) Ground plan of the same cave, made by the botanist Franz Sieber in 1821 and published in Franz Sieber, Reise nach der Insel Kreta im griechischen Archipelagus, vol. 2, Leipzig: Sorau, 1823, Tab. XIII. Similar to Figure 2B, Sieber adopted shading techniques to depict the outlines of galleries. Shading had already been used to depict elevation differences in maps of mountainous or hilly terrain.

A map made about thirty years later by the Bohemian botanist and collector Franz Sieber (1789–1844) and surveyed using compass and measuring line depicted the same cave, albeit in a completely different form (Figure 3B).Footnote ⁵⁴ Sieber had visited the Labyrinth Cave on a collecting trip to Crete in 1817. Based on his assumption that the labyrinth was an ancient quarry, he sketched in large chambers and merged natural and artificial elements, with a preponderance of the latter. Since Sieber produced the map specifically for the purpose of publication in his two-volume travel account, he was aware of the publicity it would receive. He further reinforced his argument for the cave's primarily artificial origin by adding illustrations of labyrinth motifs found on Minoan coins; his map also contains more circuits than that of Dumas.

Curiosities found underground similarly challenged the previous empirical observations made by visitors. In particular, the existence of specimens that could, potentially, be classified in a number of ways made it necessary to interrogate the description and naming of objects, resulting in various attempts to reorganize knowledge about the organic and inorganic world. As ‘hybrid objects’, speleothems (e.g. stalactites, flowstones, moonmilk) undermined the nature–culture opposition by ‘combining art and nature in form and matter’.Footnote ⁵⁵ They were collected not only by naturalists but also by locals; they were sold, placed in garden grottos, ground up and administered as medicine to pregnant women, sick people and cattle, and added to flour during food shortages. The attempts of scholars to distinguish and differentiate the sub-categories of speleothem reflect their varying uses across diverse contexts and communities. Nonetheless, speleothems were often thought to be fossils, and vice versa.Footnote ⁵⁶ For example, the Swedish chemist and naturalist Johan Gottschalk Wallerius (1709–85) claims in his treatise Mineralogia that the basis for the formation of stalactites is a fossil substance that hardens under the influence of air like an icicle in winter.Footnote ⁵⁷

Thus early cave maps prioritized the various types of minerals, fossils and artefacts encountered over geographical direction, distance and projection planes. Striking map symbols that mimicked the shape of natural specimens served to guide the reader through the labyrinths. As these were often arranged along preset underground pathways, the objects also structured the plan itself. The resulting combination of empiricism and imagination transformed these maps into wonder rooms or ‘curiosity cabinet[s] of natural science’.Footnote ⁵⁸ The shape, order and cross-referenced meaning of the specimens depicted were key to building on the prior knowledge of potential users and signalling that the map was trustworthy. By rendering hybrid specimens into more approachable objects, such as religious figures or architectural features, mapmakers made these specimens visually descriptive and comprehensible.

I would like to illustrate this hypothesis using a sketch of Baumannshöhle (Figure 4), drawn by a certain ‘Studiosus von Alvensleben’, who visited the cave in 1656 as a companion of the Saxonian theologian Gottfried Olearius (1604–85). In this map, consisting of two sheets, the curiosities found in the cave are arranged along a winding path with obstacles, emphasizing the labyrinthine structure, while the galleries are only sketched in outline.Footnote ⁵⁹ The draftsman uses features familiar to readers to represent his empirical observations in visual form. Narrow parts and climbable passages such as the ‘Roß’ (horse) are depicted as intestine-like tubes. Speleothem formations such as the ‘monk beside two other images’ and a ‘beautiful and miraculous wall of all kinds of figures’ are illustrated as anthropomorphic figures or as a cluster of very detailed objects. One eye-catching combination of stalactite and stalagmite is even depicted as a broken antique column, while other speleothems are visualized as pyramid-shaped aggregations of stones. Nevertheless, the mapmaker is well aware of possible sensory illusions and affirms the uncertainty of empirical perception underground, noting in the margin, ‘Here, [the speleothem] falsely appears to be a ghost.’

Figure 4.

Ground plan of Baumannshöhle (Saxony-Anhalt), made by the Studiosus von Alvensleben in 1656 (Landeshauptarchiv Sachsen-Anhalt, Rep. H 66, Gutsarchiv Erxleben II, Nr. 952, p. 15). The figure shows the second page of the map with the heading ‘Die mittlere Bumanßhöhle’ (The middle Baumannshöhle cave). In the upper right corner, a narrow passage is depicted, from where a visitor lowers himself with a rope. Once there, three torch-bearing companions with headgear indicate the continuation of the tour. It leads past speleothems, a ‘Brunnen’ (well), another narrow passage and a stalactite formation called ‘monk’, to the upper left edge of the map, where another visitor is depicted with a torch.

Visual narratives and map symbols

Maps aim to create order and orient the viewer via the distribution of symbolic meaning. Accordingly, early cave maps depicted the dynamics and disruption within the Earth's crust as a systemic form of disorder. Based on his readings of Kenneth Burke, the philosopher Pierre Smolarski draws a connection between the ‘rhetoric of maps’ and their organizing function, pointing out that motifs represent ‘acronyms’ and ‘pathways to orientation’.Footnote ⁶¹ The ordering of figurative symbols along pathways in cave maps resembles the rhetorical practice of ‘memoria’: an ancient technique for learning a speech by heart by systematically associating its parts with figurative signs and places. Here, motifs are essential in helping the viewer to make sense of the map and shaping their understanding of it. By simplifying the knowledge represented, they offer a clearer and more convincing narrative than the written accounts of cave visits, which are often rather confusing. The rhetoric of maps and their potential to create order only become really evident when they break new ground in their design, or else fail. Otherwise, the order they depict appears natural and convincing.

Visual narratives use recognizable symbols to communicate (non-)knowledge and make it commonly understandable. As the prevailing visual narrative used in early modern cave maps, the labyrinth motif fulfilled a central epistemic function: specifically, identifying the unknown.Footnote ⁶² Referring to Richard Yeo,

By attributing order and comprehensibility to that which is invisible to the eye, cave maps made it possible to evaluate, measure and ultimately control the unknown in nature. The labyrinth motif symbolized non-knowledge by visually contrasting centre and periphery, proximity and distance, order and mess. In this respect, the motif both marked the limits of perception and established trust. A good example of this is the depiction of the Labyrinth Cave by the Florentine humanist Cristoforo Buondelmonti (1386–c.1430), who visited the site in 1415. His manuscript ‘Descriptio insulae Cretae’, written two years later, contains the first map-like drawing of a cave incorporating orientation information.Footnote ⁶⁴ Enriched by material from ancient Greek and Roman writers and from contemporary oral accounts, Buondelmonti's description links mythological narratives with detailed information about the length and direction of the cave. His map, in contrast, quotes the visual narrative of the labyrinth by depicting several interconnected corridors that extend beyond the edge of the map format. Buondelmonti's personal observations made on site, and his detailed report, indicate that his depiction of the cave as a labyrinth is not purely imaginative; the motif itself serves to indicate the limits of knowledge.

Towards the end of the eighteenth century, the use of uniform map symbols, rather than individual objects or visual narratives, arose from fundamental shifts both in epistemological interest and in the classification of natural specimens. In the new context, experience-based knowledge had to be ranked into a hierarchy and brought into relation. What was important had to be distinguished from what was unimportant. This development made it possible to compare different caves by reading their respective maps. The use of symbols also required a higher degree of abstraction and generalization of observed natural phenomena. Some maps even included formal legends, or entire paragraphs explaining the abbreviations and clarifying the relationship between a sign and what it signified: in other words, accessible sign systems intended for a wider audience.

The loss of perceived authenticity caused by this increase in abstraction required the invention of new means of conveying empirical evidence. Likewise, the interest of naturalists and mapmakers was no longer directed solely towards the individuality of a given object, but towards its comparability to other objects too. These developments, in turn, gave rise to changes in the hierarchies of representation. Significantly more attention was paid to the representation of topographical knowledge and the exact localization of mineralogical, palaeontological or archaeological findings. Topographical features such as gradient arrows, gradient lines, differences in elevation, heights of chambers and symbols representing drops, pits or domes were particularly effective in persuading the reader to trust the information presented in the map. These features allowed conclusions to be drawn about tectonics and speleogenesis (cave formation). At the same time, collecting this kind of detailed information required the surveyor's presence on site and the taking of measurements, especially with regard to vertical difference.

A sketch of Madison's Cave in Virginia demonstrates these shifts in the scholarly interest in caves and the rhetoric of maps. The first known survey of a US cave, it was drawn by the American statesman Thomas Jefferson (1743–1826), who visited with two travel companions in 1783.Footnote ⁶⁵ After his time as governor of Virginia and following the death of his wife, Jefferson had retired from politics and devoted himself to the exploration of Virginia's natural history, even conducting systematic archaeological excavations. His lifelong interest in caves may have been driven by his convictions about the potential production of saltpetre from soils. Saltpetre was essential to the manufacture of gunpowder, so finding new methods of production was an important question of national defence.Footnote ⁶⁶ Jefferson's sketch supports his hydrological hypotheses. Although he mentions the cave's ‘elegant drapery’, stalactites and massive columns in his description – later published, together with a generalized version of the map, in Notes on the State of Virginia – they do not appear on the map itself.Footnote ⁶⁷ Rather, Jefferson's sketch indicates two water basins ‘of unknown extent’ at the rear of the cave as well as the difference in elevation of the galleries, marked using gradient arrows and numerical data in feet and degrees. Although Jefferson observed that the basins were at the same elevation as a nearby river, he concluded from the clarity of the water and the constant water table that they were ‘probably one of the many reservoirs with which the interior parts of the earth are supposed to abound and yield supplies to the fountains of water distinguished from others only by being accessible’.Footnote ⁶⁸ Jefferson's map symbols, therefore, did not have a purely visual function. In fact, they were abstractions of topographic knowledge based on measurements and were embedded in the context of a specific scientific argument, even though the surveying techniques and tools employed often did not and could not provide accurate data.

Surveying and accuracy

Today's surveyors face the same problems they did three hundred years ago. They typically underestimate the size of large halls, while overestimating that of smaller chambers. Besides restricted visibility, vertical or narrow sections that are difficult or impossible to traverse can also cause significant errors of measurement. This becomes particularly obvious when a loop within a cave is surveyed, or when two traverse lines, drawn on the basis of a sequence of measurements along a cave passage, happen to cross. In addition, equipment such as measuring tools and instruments – which have to be dragged behind during the descent – are exposed to extreme conditions that affect their precision. Reliable observation depends on conditions that cannot be guaranteed in practice. In short, cave maps claim a degree of accuracy that is impossible to achieve.

A surviving ink-and-wash drawing from 1749 by the Italian engineer and painter Carlo Beduzzi (c.1710–c.1770), depicting the artist in the process of making a sketch of the Magdalena Cave in Slovenia for a factual report to the Vienna court (Figure 5), provides an insight into early modern surveying practices. Torchbearers are placed to illuminate an extensive hall within the cave. An assistant winds a tape on a reel, while the surveyor, dressed in a frock coat, knee breeches, tricorn hat and wig, sketches a draft on a large sheet: the back of another assistant serves as his drawing board. In narrow or steep cave sections, the surveying process was undoubtedly much less orderly and careful than Beduzzi's picture suggests.

Figure 5.

Detail of an ink-and-wash drawing of Magdalena Cave (Slovenia) in Joseph Nagel, ‘Beschreibung deren auf allerhöchsten Befehl … untersuchten, in dem Herzogthume Crain befindlichen Seltenheiten der Natur’ (Austrian National Library, Ms. 7854, Tab. VII). Its painter, Carlo Beduzzi, an Italian engineer working in Vienna, accompanied the imperial mathematician Joseph Anton Nagel (1717–94) on a trip to Carniola. Nagel visited caves and other natural wonders in lower Austria, Styria, Carniola and Moravia in 1747 and 1748 on behalf of Emperor Franz Stephan (1708–65). Beduzzi made no less than four maps and nineteen drawings of caves for one of the two reports to the emperor, including this drawing showing the impressive stalactites of Magdalena Cave. Fitting for an official report to the court, the speleothems were depicted quite realistically. Note: see Johanna Schönburg-Hartenstein, Josef Anton Nagel: Ein Direktor des physikalischen Kabinettes, Vienna: Austrian Academy of Sciences, 1987; Trevor R. Shaw and Alenka Čuk, Slovene Caves and Karst Pictured 1545–1914, Ljubljana: Založba ZRC, 2012, p. 88.

Although surveying methods that enabled accurate scale mapping were developed, mainly in the seventeenth century, it is doubtful that these were applied in cave environments.Footnote ⁶⁹ This is also true of well-established mine surveying techniques, which were only of limited use in natural cavities. In particular, surveying conditions in mine tunnels following the course of ore veins – which tended to be rather easier to access – were significantly different to the geographies of natural caves, which were winding and hard to traverse. There, the traditional method of triangulation was at a disadvantage compared to traverse surveying, even if we allow for a higher degree of inaccuracy. There is also little evidence of professional mine surveyors engaged in mapping before 1800. Although a number of tools and instruments used in early modern mine surveying – such as the measuring rod, miner's level, compass and plumb line – are mentioned in reports of early modern cave visits, it is uncertain to what extent these were actually used in practice.

The most common tool was certainly the measuring cord, supplemented as necessary with a hand-bearing magnetic compass, angle meter and plumb line. In many cases, just a cord and compass may have been used for taking measurements along more or less continuous traverse lines. Surveying steep sections was a tricky and unsafe task that could easily produce contradictory results. Determining the angle of inclination of these sections was particularly challenging. Since pits in caves are not always vertical, plumb lines were of limited use. Angle meters also had high error rates due to difficulties in targeting. As inaccuracies in measuring depth added up in the course of a traverse survey, vertical data in cave surveys were marked by a high margin of error and uncertainty.

The challenge of translating the empirical data obtained by surveyors into cartographic codes was not only a practical one, but also epistemological. Mapmakers in the early modern period had to use specific strategies to deal with questions of accuracy and to communicate trustworthiness to potential users. While some mapmakers openly admitted the uncertainty of their maps by giving them titles such as ‘eye-draught’, others asserted their credibility by including depictions of measuring tools. Before the appearance of scale bars, human figures were depicted as a comparative sizing method. The inclusion of these figures, shown holding lights in the cave's corridors – they sometimes included the mapmaker himself – conveyed a sense of authenticity. During the eighteenth century, mapmakers tried to enforce their claims to accuracy by illustrating epicartographic elements as scale bars and by referring in various ways to surveying practices.Footnote ⁷⁰ Within a given map, the scale of the objects depicted also varied in accordance with the mapmaker's sense of the user's expectations. Areas of particular significance to clients might be drawn on a larger scale, while other, less interesting, galleries might be left out altogether. Other convincing epicartographic elements such as north arrows or compass roses would be depicted on the map even if measuring tools were obviously not used when exploring the cave.

Projection planes and verticality

The complex three-dimensional topography of cave systems presented early modern mapmakers with a considerable challenge. In particular, multiple over- and underlays of corridors were difficult to depict in a two-dimensional drawing. The need to present whole cave systems and natural specimens in a traceable and comprehensible manner gave rise to the development of consistent projection planes in cave surveying, such as ground plans, longitudinal sections and cross-sections.

Originating as drawings incorporating orientation information, the basis of cave maps gradually shifted from the subjective perspective chosen by a given observer to the kind of abstract projection plane already used in the topographical maps of the period. Perspective map projections functioned as an intermediate stage between the two, providing a three-dimensional overview while still enabling the reader to inhabit the mapmaker's chosen viewpoint.Footnote ⁷¹ This change in the use of projection planes also threw up a number of hybrid forms.Footnote ⁷² In order to give an overview of phenomena that could not be represented solely by a ground plan or sections, the draftsmen combined different projection planes within a single map and illustrated the galleries’ outlines multidimensionally. This was intended to reinforce the credibility of the sketch.

The interpenetrating power of maps and their ability to render a mountain transparent were key to persuading the reader to trust the information presented in them. In particular, the switch from horizontal to vertical projection planes exposed areas of the Earth's subsurface that were previously inaccessible to human perception and aroused interest in their examination. As naturalists gradually came to understand the underground in terms of the Earth's development over time, they experienced, in the words of the German naturalist Georg Christoph Lichtenberg (1742–99), ‘phenomena that raise questions such as How? Why? and Whence?’Footnote ⁷³ Growing interest in geological and archaeological phenomena made verticality an increasingly important dimension of knowledge acquisition, although the measurement of vertical dimensions remained strikingly unreliable and produced wildly varying results. While early cave maps were mostly ground plans and did not show vertical differences, such as pits, steps and chimneys, longitudinal sections of caves soon became common map projection planes.

Longitudinal sections were especially important: they opened up new opportunities for viewing phenomena in comparison and bringing them into the context of a given scientific argument. Drawing a longitudinal section, early mapmakers were embarrassed by so much blank space between the surface of the Earth and the roof of the cave, and they started to use this previously neglected area to enhance the map's scientific value. This allowed them to make an explicit connection between the topography of the cave, the findings made there and the conditions on the Earth's surface: for example, drawing in fissures in order to attribute the occurrence of stalactites to the intrusion of vadose water from subsurface soils.Footnote ⁷⁴ As I will discuss in more detail in the following section, cross-sections of cave passages, which were sometimes made in supplement to ground plans and longitudinal sections, allowed the mapmaker to draw connections between the location, type and frequency of findings and the process by which the cave is thought to have formed.

A fascinating example of an early longitudinal section and its techniques of persuasion is a 1719 survey of Demänovská Ice Cave in Slovakia, where ice was routinely cut for the Vienna court. A detail of the map is provided in Figure 6. It was made in 1719 by the naturalist Georg Buchholtz (1688–1737), rector of the Protestant lyceum in Kežmarok, and possibly commissioned by the Hungarian polymath Mátyás Bél (1684–1749). Buchholtz also printed the sketch in the style of a formal, scholarly map in his 1723 country description Hungariae antiquae et novae prodromus.Footnote ⁷⁵ Since this map is the only cartographical representation included in the treatise apart from a single topographical map, it was clearly intended to make an impression on the reader. Although it is questionable whether Buchholtz actually used surveying tools, these are depicted together with a scale bar below the title cartouche. Two scale figures with torches shown standing inside the cave, and the figures of two scholars approaching its entrance, accompanied by a porter, underpin the authenticity of the depiction. Ice figures, speleothems and fossils encountered underground are sometimes illustrated in anthropomorphic form, and about fifty abbreviations, explained at the edge of the map, offer detailed information on the occurrence and origin of the curiosities. Buchholtz also uses the map to furnish evidence of the location where he had found some supposed ‘dragon bones’ (mammoth fossils), which he sent as a gift to the art and natural cabinet in Dresden.Footnote ⁷⁶ His choice of a vertical projection plane certainly makes the course of the cave appear longer, easier to grasp and therefore more impressive than it would appear on a ground plan due to its labyrinthine topography. However, this choice meant that the map was far from suitable for helping other visitors find their way inside the cave. Probably unsure how to fill the white space between the surface of the Earth and the roof of the cave, Buchholtz depicts the stratification of the Earth using a rockface pattern inhabited by single, anthropomorphic faces. In contrast to later mapmakers, however, he does not sketch in a hypothetical geological relationship between the course of the cave and the surface of the Earth.

Figure 6.

Detail of the longitudinal section of Demänovská Ice Cave (Slovakia), surveyed by Georg Buchholtz in 1723, engraved by Sámuel Mikoviny and published by Mátyás Bél (ed.), Hungariae antiquae et novae prodromus, Nuremberg: Monath, 1723, p. 141 (Sammlung Woldan, Austrian Academy of Sciences). The two wind roses indicate the direction of expansion of the upper and lower cave entrances.

The use of abstract projection planes certainly made maps less accessible to inexperienced readers. However, the consistency of projection planes and the possibility of comparing different sketches were key to establishing credibility, and met the growing need to examine different natural phenomena in a comparative way. Due to the complex three-dimensional structure of caves, ground plans were extremely limited in their ability to render vertical differences such as pits, or how far one gallery lies above another. In contrast, longitudinal sections were unsuitable for depicting labyrinth cave systems, and broadly useless in pathfinding due to the lack of directional information. Therefore mapmakers came to adopt a technique already used in architectural drawings, in which ground plan, vertical section and profile view were all created in the same process.Footnote ⁷⁷

Localizing findings of natural-historical significance

According to Nicolaas Rupke, caves ‘were believed to be pervasive and primordial features, present since the birth of the earth as a planet, providing essential information as to the manner of its origin’.Footnote ⁷⁹ At the subterranean level, the beginnings of the Earth's chronology became visible in fossil deposits, speleothems and different layers of rock or soil. The human lack of awareness of immeasurable periods of the Earth's history and the growing realization of ‘geological deep time’ transformed caves into enlightening and challenging sites of natural history.Footnote ⁸⁰ Martin Rudwick's interpretation of the well-known caricature The Hyaena's Den at Kirkdale, which depicts the renowned English geologist and palaeontologist William Buckland (1786–1856) crawling into a cave and encountering living hyenas from the prediluvial period, illustrates how naturalists broke the ‘epistemic barrier that separated the observable present from the prehuman past’.Footnote ⁸¹ Formerly inaccessible periods materialized under the scholars’ feet, were made transparent by vertical sections, and thus became a topic for scientific reasoning.

Cave maps did not only provide insights into the places where the discoveries were made. Increasingly, they also functioned as veritable archives of natural history, where the deposition of fossils or mineral formations could be seen on display. The aforementioned map of Baumannshöhle, included in Leibniz's posthumously printed Protogaea, sheds light on the fundamental nature of evidence building in drawing conclusions about the history of the Earth. Unlike the original map by Hermann von der Hardt, Leibniz's revised and supplemented version was not a tool for other visitors, indicating the underground curiosities most worth seeing. Illustrating the shift in function from merely locating specimens to placing them in a causal relationship with natural history, the map was intended to designate the sites of specimen discovery and to differentiate and highlight the findings through detailed, naturalistic representations.Footnote ⁸²

During his visit to the cave, Leibniz collected fossils and speleothems. His descriptions of these findings identify similarities with the bones of animals or the minerals found in mines. Likewise, he saw in the structure and stratification of speleothems ‘something like the rings that designate years in trees … since nature rested intermittently and then resumed her work again with a new inundation’.Footnote ⁸³ In Leibniz's map, the image of one distinctive fossil specimen, enlarged and placed at the centre, alludes to his hypothesis mentioned in Protogaea that specimens ‘did not arise in those places but came from the ocean, carried there by the violence of the waters’.Footnote ⁸⁴ Remains of ‘sea monsters and of other animals’,Footnote ⁸⁵ they were swept, together with those of terrestrial animals, into caves by a great flood as its waves retreated into the Earth's interior. According to the valuable work of Claudine Cohen and Andre Wakefield, Leibniz's ideas about the origins of fossils changed considerably during his lifetime; however, caves as sites of discovery remained crucial to his thought.Footnote ⁸⁶ Although the Baumannshöhle map was not in itself intended to support a particular hypothesis, his detailed comparison of various specimens and his specific identification of fossils enabled a better understanding of universal principles, contributing to the generalization of local knowledge.

As well as cartographic features and the depiction of surveying practice, who made a given map and where it was published were crucial factors in establishing its credibility. Maps often furnished evidence for the hypotheses of their authors, and as such were closely tied to the argumentative context of their publication. Only rarely did naturalists refer in their hypotheses to maps made by or for others. The reputation of the editor and the scientific recognition of the publication medium contributed to the perceived trustworthiness of the depiction. This explains why maps that were not printed in scientifically accredited treatises, country descriptions or journals were only afforded limited consideration in the scientific debate. In individual cases, a scholar's credibility and authority might even allow him to present his hypothetical map as concrete evidence for his argument, without actually carrying out any fieldwork.

But implicit acceptance was far from guaranteed. Serving as a key foundation for his hypothesis about the origin of fossils, the cave maps printed by William Buckland in his Reliquiae Diluvianae were openly criticized for being a ‘hypothetical presentation’ rather than the ‘result of an investigation’.Footnote ⁸⁷ Buckland's sketches, mapped by the geologist Thomas Webster (1773–1844) using a vertical projection plane, stimulated lively scholarly debate.Footnote ⁸⁸ Evidence found in caves had drawn considerably more attention from naturalists after Johann F. Esper's (1732–81) groundbreaking identification of rich fossil deposits in Gailenreutherhöhle (Franconia) as the remains of extinct animals such as cave bears.Footnote ⁸⁹ One such naturalist was the anatomist John Hunter (1728–93), who argued in his paper presented to the London Royal Society that the ‘vast accumulation’ of fossils in Gailenreutherhöhle required the presence of living animals in the cave for

Building on Hunter's hypothesis that fossil findings in caves were the remains of animals that had once lived there, Buckland attributed a larger role to a prehistoric flood. Although he argued that, in most cases, the waters did not wash the bones into the cave, he specified that they were responsible for covering the fossils with mud.Footnote ⁹¹ To reinforce this statement, Buckland's maps, such as the longitudinal section of Gailenreutherhöhle, do not depict successive layers of cave deposits but show the specimens as being of similar age.Footnote ⁹² The Gailenreutherhöhle map clearly distinguishes between fossiliferous strata and the speleothems that formed above them, indicating that the latter came after the deluge. Although he himself had visited Gailenreutherhöhle, Buckland also depicts layered deposits and stratifications of findings that had not, in fact, been verified in situ. Combining a vivid sense of the fascination of the underground with the depiction of caves as palaeontological sites, Buckland's maps became powerful epistemic tools capable of advancing a hypothesis and apparently confirming it at the same time.

Reasoning about speleogenesis

The variety of early modern concepts about speleogenesis cannot be understood in isolation from scientific ideas about fossil and speleothem formation and how these specimens came to be found in the earth. Some of these hypotheses were tied to universal exploratory models of the Earth's formation; others were made in the course of personal observations on site, but did not claim universal validity. Alongside a gradual shift from catastrophic to actualistic viewpoints, greater significance was accorded to the development of universally valid models of speleogenesis, which combined different factors such as corrosion, erosion, volcanism, gases and tectonic causes. Trevor Shaw, who has carefully collected and categorized the various historical theories about cave formation, classifies them according to the following factors: tectonic origins (uplift, collapse, underground expansion); drains for the deluge; gas bubbles in limestone; erosion of soft material or limestone; solution by thermal, vadose or phreatic waters; volcanism; and atmospheric corrosion.Footnote ⁹³

Arising as they did from a general interest in natural-history specimens, it was relatively late before cave maps were employed in causal reasoning about speleogenesis. The reason for this was that longitudinal sections tended to focus on depicting cave deposits, whereas ground plans were more suitable for pathfinding. Cross-sections of galleries, which were important for attempts to reconstruct the process of speleogenesis, did not become a common feature of cave maps until the nineteenth century. Often, naturalists used artistic drawings instead of maps to explain the formation of a given cave.Footnote ⁹⁴ Others employed diagram-like maps showing supposed subterranean water reservoirs and channels to explain the fluctuating level of intermittent lakes and springs.Footnote ⁹⁵

For many mapmakers in this period, it was more important to give a visual representation of a particular scientific argument than it was to make a claim to accuracy. In particular, the shape of the galleries’ outlines and the way in which they were visualized on a given map underpinned the author's own hypothesis about how the cave was formed.Footnote ⁹⁶ One of the few pre-1800 ground plans to serve as an argument about speleogenesis is a 1753 map of the lava tube cave Surtshellir in Iceland. Based on a survey by the Icelandic naturalist Eggert Ólafsson (1726–68) and the physician Bjarni Pálsson (1719–79), the map was not printed until 1772 in Ólafsson's posthumously published and widely translated account Reise igiennem Island.Footnote ⁹⁷ Entrusted by the Royal Danish Academy of Sciences with the task of studying the island's topography and mineral deposits, the two naturalists travelled their homeland for several years. In the course of their research, they also surveyed Surtshellir, a cave that was of particular interest to scholars at the time. Stalactites collected there had been circulating among scientists since the seventeenth century, stimulating the development of widely differing hypotheses about the nature of their origins. In his description, Ólafsson pays specific attention to the cave's structure, such as walls, shelves, horizontal benches, speleothems and the glazed ceramic appearance of the walls. He attributes these features to the effect of lava flowing through the cave. In order to support this hypothesis about the cave's volcanic origin, the map shows an exaggeratedly worn and smoothed texture in the outline of the galleries, as if shaped by a river. This resulted in

While surveys like those of Surtshellir served as evidence both for the detailed observations made by the draftsmen on site and for the credibility of the arguments on which they were based, other maps themselves stimulated the development of new hypotheses. This is especially true for cross-sections of cave passages. Providing evidence of mechanical erosion, dissolution and tectonic processes, these maps inspired scholars to consider a combination of different factors in cave formation. This insight became clear to the French engineer and mathematician Nicolas Brémontier (1738–1809), in later years inspector general of bridges and roads, when he surveyed Rouffignac Cave in Dordogne, now famous for its prehistoric paintings.Footnote ⁹⁹ Most likely made in 1765, the map comprises a surprisingly accurate ground plan and no less than twenty-seven cross-sections of passages. The highly irregular shape of the cross-sections, depicted side by side and aligned along a common datum line, led Brémontier to hypothesize about speleogenesis in his later report to the Academy of Caen. According to him, some parts of the cave were primarily formed by stream erosion, after an earthquake caused fissures in the rock and allowed the water to infiltrate; other parts developed through rock corrosion caused by air: ‘The walls and roof provide a tangible proof of this … This difference in hardness has resulted in pendants, domes, partitions … hollows and projections.’Footnote ¹⁰⁰ By understanding speleogenesis as a complex interaction of different geological forces, such as mechanical erosion, and tectonic and corrosive action, scholars and their cave maps were able to cast doubt on previously accepted universal models of the Earth's history, despite the uncertainty of the evidence from which they worked.