Construction of Fort Jefferson (named after the third U.S. President, Thomas Jefferson) was finally begun on Garden Key in December 1846, under the supervision of 2nd Lt. Horatio Wright, after plans drawn up by Lt. Montgomery C. Meigs were approved in November. Meigs' plans were based on a design by Joseph Totten. Chief of Engineers Totten eventually visited the fort in 1855, accompanying Louis Agassiz.
The new fort was built so that the existing Garden Key lighthouse and the lighthouse keeper's cottage were contained within the walls of the fort. The lighthouse continued to serve a vital function in guiding ships through the waters of the Dry Tortugas Islands until the current metal light tower was installed atop an adjacent wall of the fort in 1876. The original brick lighthouse tower was taken down in 1877.
Fort Jefferson can be reached by a daily ferry from Key West, as well as by chartered seaplane and private boat. As a national park, primitive camping is permitted on the beach. Visitors by ferry typically spend four hours on the island, which is enough time for a guided tour of the fort, lunch on the boat and a swim (snorkel equipment provided) on the reef. Within the fort are a museum and bookstore.
According to wikipedia