Geologic Background - Flat and Wet
For most of its geologic history, Florida was under water. The shells of millions of sea animal form the layers of limestone that blanket the State. The peninsula rose above sea level about 20 million years ago. Even then, the southern portion remained largely submerged, until the buildup of coral and sand around its rim blocked out the sea, leaving dense marine vegetation to decay and form the peaty soil of the present-day Everglades.
Dependent on rain for freshwater, the subtropical stretch of the peninsula receives 40 to 65 inches a year. But this flat, porous limestone land has little surface storage capacity, and after evaporation, transpiration, and runoff, only a fifth of the rainwater remains to recharge underlying aquifers and shallow lakes.