Fort Jefferson is a massive but unfinished coastal fortress. It is the largest masonry structure in the Americas, and is composed of over 16 million bricks. The Dry Tortugas are part of Monroe County. The fort is located on Garden Key in the lower Florida Keys within the Dry Tortugas National Park, about 70 miles (110 km) west of the island of Key West. A series of engineering studies and bureaucratic delays consumed 17 years, but the construction of Fort Jefferson (named after the third President, Thomas Jefferson) was finally begun on Garden Key in 1846. The new fort would be built so that the existing Garden Key lighthouse and the lighthouse keeper’s cottage would be contained within the walls of the fort. The lighthouse would continue to serve a vital function in guiding ships through the waters of the Dry Tortugas Islands until the current metal light tower was installed atop an adjacent wall of the fort in 1876. The original brick lighthouse tower was taken down in 1877.
The design called for a four-tiered six-sided 1000 heavy-gun fort, with two sides measuring 415 feet, and four sides measuring 564 feet. 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. Living quarters for soldiers and officers, gunpowder magazines, storehouses, and other buildings required to maintain the fort were located on the parade ground inside the fort’s massive brick walls. The Army employed civilian machinists, carpenters, blacksmiths, masons, general laborers, the resident prisoner population, and slaves to help construct the fort. By 1863, during the Civil War, the number of military convicts at Fort Jefferson had increased so significantly that slaves were no longer needed. At the time, there were 22 black slaves employed on the project. 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.
On January 4, 1935, President Franklin D. Roosevelt, who visited the area by ship, designated the area as Fort Jefferson National Monument. It was listed on the National Register of Historic Places on November 10, 1970. On October 26, 1992 the Dry Tortugas, including Fort Jefferson, was established as a National Park. The islands still do not exhibit any standing fresh water or even seasonal streams, hence the name “dry”. Owing to the potential difficulties of survival in such conditions, one of these islands was used as the location for filming a military survival film used to train aircraft personnel.
Fort Jefferson can be reached by a daily ferry from Key West, as well as by chartered seaplane and private yacht. As a national park, camping is permitted on the beach. Visitors by ferry typically spend 4 hours on the island, which is enough time for a guided tour of the fort, lunch on the boat, and a swim (snorkel equipment provided) on the reef. The island has a gift shop but no food is available for visitors.