Fort Jefferson, Sea fort in Dry Tortugas, United States.

Fort Jefferson is a hexagonal sea fort on Garden Key in the Dry Tortugas, Florida, built from more than 16 million bricks across roughly 16 acres. The two-story structure forms a ring of casemates around a central parade ground, surrounded by thick walls and bastions.

Construction of Fort Jefferson began on Garden Key in December 1846 under 2nd Lt. Horatio Wright, following plans by Lt. Montgomery C. Meigs based on a design by Joseph Totten. Enslaved workers hired from Key West and other parts of Florida formed the backbone of the labor force until 1863. The fort was still half complete in 1860 and was never finished; during the Civil War it held deserters and political prisoners, including Samuel Mudd, who was pardoned in 1869 for halting a yellow fever epidemic.

The fort is named for Thomas Jefferson, the third U.S. president, but the design came from engineers Joseph Totten and Montgomery C. Meigs. During the Civil War it held political prisoners and deserters, among them Samuel Mudd, the doctor convicted for treating Lincoln's assassin; a yellow fever epidemic in 1867 killed the medical staff before the soldiers, and Mudd took over and treated the sick, earning a pardon in 1869.

Access is by ferry or seaplane from Key West, with the boat trip taking around 2.5 hours. The park has no shops or restaurants, so visitors should bring water, food, and sun protection.

The fort never fired a shot in battle and remained unfinished at war's end, its brick walls already outdated. Today it hosts thousands of seabirds that nest on the walls and flutter through the empty passageways.