Fort Jefferson is a massive but unfinished coastal fortress. It is the largest brick masonry structure in the Americas, and is composed of over 16 million bricks. The building covers 16 acres (6.5 ha). Among United States forts, only Fort Monroe in Virginia and Fort Adams in Rhode Island are larger.
The fort is located on Garden Key in the lower Florida Keys within the Dry Tortugas National Park, 68 miles (109 km) west of the island of Key West. The Dry Tortugas are part of Monroe County, Florida, United States.
Fort Jefferson was built to protect one of the most strategic deep water anchorages in North America. By fortifying this spacious harbor, the United States maintained an important “advance post” for ships patrolling the Gulf of Mexico and the Straits of Florida. Nestled within the islands and shoals that make up the Dry Tortugas, the harbor offered ships the chance to resupply, refit, or seek refuge from storms.
The location of the Tortugas along one the world’s busiest shipping lanes was its greatest military asset. Though passing ships could easily avoid the largest of Fort Jefferson’s guns, they could not avoid the warships that used its harbor.