Lake Okeechobee (US /oʊkiˈtʃoʊbi/), locally referred to as "The Lake", "Florida's Inland Sea", or "The Big O", is the largest freshwater lake in the state of Florida. It is the seventh largest freshwater lake in the United States and the second largest freshwater lake (the largest being Lake Michigan) contained entirely within the contiguous 48 states. However, it is the largest freshwater lake completely within a single one of the lower 48 states. Okeechobee covers 730 square miles (1,900 km2), approximately half the size of the state of Rhode Island, and is exceptionally shallow for a lake of its size, with an average depth of only 9 feet (2.7 metres). The lake is divided between Glades, Okeechobee, Martin, Palm Beach, and Hendry counties. All five counties meet at one point near the center of the lake.
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 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, the wetlands built up the layers of peat rapidly enough (reaching 4-to-4.3-metre (13 to 14 ft) thick) to form a dam, until the lake overflowed into the Everglades. At its capacity, the lake holds 1 trillion gallons of water and is the headwaters of the Everglades.