Dry Tortugas (tôrtōˈgəz) [key], island group in the Gulf of Mexico, off S Fla., 60 mi (97 km) W of Key West. Named by the Spanish explorer Ponce de León in 1513, the islands later became a pirate base. They are famous for their bird and marine life. Loggerhead Key is the largest island. Centered on Garden Key is Dry Tortugas National Park (64,701 acres/26,195 hectares). The park caontains Fort Jefferson (1846), the largest all-masonry fort in the Western Hemisphere, which served as a prison until 1874. Designated Fort Jefferson National Monument in 1935, it was renamed and became a national park in 1992. See National Parks and Monuments (table). The island group is also known as Tortugas.