Dry Tortugas National Park preserves Fort Jefferson and the Dry Tortugas section of the Florida Keys. The park covers 101 mi² (262 km²), mostly water, about 68 statute miles (109 km) west of Key West in the Gulf of Mexico.
The first European to see the islands was Juan Ponce de León, who visited in June 21, 1513. Ponce de León caught 160 sea turtles there and subsequently referred to the islands as the “Tortugas” (turtles). They are called Dry owing to the absence of surface fresh water on the island.
The design called for a three-tiered six-sided 420 heavy-gun fort, with two sides measuring 325 feet (99 m), and four sides measuring 477 feet (145 m). The walls met at corner bastions, which are large projections designed to allow defensive fire along the faces of the walls they joined. The heavy guns were mounted inside the walls in a string of open casemates, or gunrooms, facing outward toward the sea through large openings called embrasures. Fort Jefferson was designed to be a massive gun platform, impervious to assault, and able to destroy any enemy ships foolhardy enough to come within range of its powerful guns.
Fort Jefferson’s peak military population was 1,729. In addition, a number of officers brought their families, and a limited number of enlisted personnel brought wives who served as laundresses (typically four per company). There were also lighthouse keepers and their families, cooks, a civilian doctor and his family, and others. In all, there were close to 2,000 people at Fort Jefferson during its peak years.
In order to support such a large population in an area lacking fresh water, an innovative system of cisterns was built into the walls of the fort. Sand-filled columns were placed at regular intervals in the inner walls, spanning their height from the roof to the foundation. The columns were intended to filter rainwater from the rooftop for long-term storage in a series of underground chambers. However, the system was never used in practice, as the enormous weight of the outer walls caused them to subside; this created cracks in the cisterns, allowing seawater to contaminate the fresh water supply.