Florida is a huge plateau, much of it barely above sea level. The highest point in the state is believed to be a hilltop in the panhandle, 345 ft (105 m) above sea level, near the city of Lakewood, in Walton County. No point in the state is more than 70 mi (113 km) from saltwater.
Most of the panhandle region is gently rolling country, much like that of southern Georgia and Alabama, except that large swampy areas cut in from the Gulf coast. Peninsular Florida, which contains extensive swampland, has a relatively elevated central spine of rolling country, dotted with lakes and springs. Its east coast is shielded from the Atlantic by a string of sandbars. The west coast is cut by numerous bays and inlets, and near its southern tip are the Ten Thousand Islands, a mass of mostly tiny mangrove-covered islets. Southwest of the peninsula lies Key West, which, at 24°33′ N, is the southernmost point of the US mainland.
Almost all the southeastern peninsula and the entire southern end are covered by the Everglades, the world's largest sawgrass swamp, with an area of approximately 5,000 sq mi (13,000 sq km). The Everglades is, in a sense, a huge river, in which water flows south–southwest from Lake Okeechobee to Florida Bay. No point in the Everglades is more than 7 ft (2 m) above sea level. Its surface is largely submerged during the rainy season—April to November—and becomes a muddy expanse in the dry months. Slight elevations, known as hammocks, support clumps of cypress and the only remaining stand of mahogany in the continental US. To the west and north of the Everglades is Big Cypress Swamp, covering about 2,400 sq mi (6,200 sq km), which contains far less surface water.
Lake Okeechobee, in south-central Florida, is the largest of the state's approximately 30,000 lakes, ponds, and sinks. With a surface area of about 700 sq mi (1,800 sq km), it is the fourth-largest natural lake located entirely within the US. Like all of Florida's lakes, it is extremely shallow, having a maximum depth of 15 ft (5 m), and was formed through the action of groundwater and rainfall in dissolving portions of the thick limestone layer that underlies Florida's sandy soil. The state's numerous underground streams and caverns were created in a similar manner. Because of the high water table, most of the caverns are filled, but some spectacular examples thick with stalactites can be seen in Florida Caverns State Park, near Marianna. More than 200 natural springs send up some seven billion gallons of groundwater per day through cracks in the limestone. Silver Springs, near Ocala in north-central Florida, has the largest average flow of all inland springs, 823 cu ft (23 cu m) per second.
Florida has more than 1,700 rivers, streams, and creeks. The longest river is the St. Johns, which empties into the Atlantic 19 mi (42 km) east of Jacksonville: estimates of its length range from 273 to 318 mi (439 to 512 km), an exact figure being elusive because of the swampy nature of the headwaters. Other major rivers are the Suwannee, which flows south from Georgia for 177 mi (285 km) through Florida and empties into the Gulf of Mexico; and the Apalachicola, formed by the Flint and Chattahoochee rivers at the Florida-Georgia border, and flowing southward across the panhandle for 94 mi (151 km) to the Gulf. Jim Woodruff Lock and Dam is located on the Apalachicola about 1,000 ft (300 m) below the confluence of the two feeder rivers. Completed in 1957, the dam created Lake Seminole, most of which is in Georgia.
More than 4,500 islands ring the mainland. Best known are the Florida Keys, of which Key Largo—about 29 mi (47 km) long and less than 2 mi (3 km) wide—is the largest. Key West—less than 4 mi (6 km) long and 2 mi (3 km) wide—a popular resort, is the westernmost.
For much of the geological history of the US, Florida was under water. During this time, the shells of countless millions of sea animals decayed to form the thick layers of limestone that now blanket the state. The peninsula rose above sea level perhaps 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.