The Dry Tortugas are a group of islands located some 120 km (75 mi) west of Key West, Florida; they form the western end of the Florida Keys in the Gulf of Mexico. Like the Keys, the Dry Tortugas are formed primarily of coral reefs over older limestone formations. The islands were named "Dry Tortugas" upon discovery by Ponce de Leon in 1513 – "tortugas" means turtles in Spanish, and the islands are "dry" as no fresh water is found on them. Accessible only by boat or seaplane, the islands nevertheless have been designated a national park and are visited by hundreds every year. This view highlights three islands in the group: Bush Key, Hospital Key, and Garden Key – the site of hexagonal Civil War-era Fort Jefferson. Image courtesy of NASA.