Nicaragua is the largest (geographical area) country in Central America, but has the lowest population per hectare. For the last half-century its rich natural resources—timber, fruits, and minerals—were exploited by North American and European corporations without any regard for basic ecological principles. Furthermore, early in the 1950s, cotton was introduced and became the main source of foreign exchange. This led to massive use of pesticides, and construction of pesticide manufacturing plants, causing disastrous pollution by mercury of Lakes Managua and Nicaragua. It also forced migration of thousands of local subsistence farmers from the lowland cotton-producing areas to the hills where the farmers' ‘slash and burn’ techniques exacerbated the damage already done to the forests by foreign companies.