High Quality Content by WIKIPEDIA articles! Fort Jefferson is a massive but unfinished coastal fortress. It is the largest masonry structure in the Western Hemisphere, and is composed of over 16 million bricks. The Dry Tortugas are part of Monroe County, Florida, United States. 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.