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.