The Dry Tortugas are a National Park about 70 miles west of Key West. The major island contains Fort Jefferson, which just about fills the whole atoll. This fort is said to be the second largest brick edifice in the world, after the Great Wall of China. Built by the North during the Civil War, northern prisoners were kept there. Perhaps the most famous inmate was Samuel Mudd, who mended Lincoln-assassin John Booth's broken leg.