You have clicked on Lake Okeechobee.
Lake Okeechobee sits in a shallow geological trough that also underlies the Kissimmee River Valley and the Everglades. The trough is underlain by clay deposits that compacted more than the limestone and sand deposits did along both coasts of peninsular Florida. Until about 6,000 years ago, the trough was dry land. As the sea level rose, the water table in Florida also rose, and rainfall increased. From 6,000 to 4,000 years ago wetlands formed in much of what is now the lake, building up peat deposits. Eventually the water flow into the area created a lake, drowning the wetlands. Along what is now the southern edge of the lake, however, the wetlands built up the layers of peat rapidly enough (eventually reaching 13 feet (4.0 m) to 14 feet (4.3 m) thick) to form a dam, holding back the water in the lake until it rose high enough to the flow over the shore into the Everglades. At its capacity, the lake holds 1 trillion gallons (1,000,000,000,000) of water and is the headwaters of the Everglades.