Fort Jefferson on Garden Key Supply and subsidence problems and the Civil War delayed construction. The fort was never completed because of fears that additional bricks and cannon would cause further settling and place more stress on the structure and the cistern system.

Fort Jefferson, the massive coastal fortress in the Dry Tortugas, was never finished primarily due to a combination of technological obsolescence and structural engineering failures. Construction began in 1846, but by the time of the American Civil War, the development of rifled cannonry meant that the fort's thick masonry walls could be easily breached, rendering the traditional "brick-and-mortar" fortification design outdated. Furthermore, the sheer weight of the 16 million bricks caused the fort to sink into the coral sand foundation, leading to significant cracking in the walls and cisterns, which contaminated the essential freshwater supply. The remote location also made the logistics of transporting materials and labor prohibitively expensive. In the late 19th century, the fort was plagued by frequent outbreaks of yellow fever, further discouraging continued investment. Although it served as a prison (most famously for Dr. Samuel Mudd) and a coaling station, the U.S. government eventually halted construction in 1875. In 2026, it stands as a breathtaking National Park, preserved in its "half-finished" state as a testament to 19th-century military ambition and the challenges of building in a fragile marine environment.