A colossal unfinished 19th-century brick fortress filling an entire island in the middle of the Gulf, ringed by a moat, with…A colossal unfinished 19th-century brick fortress filling an entire island in the middle of the Gulf, ringed by a moat, with the healthiest coral in the Florida Keys growing on the moat wall and the surrounding shoals. Snorkel straight off the beach onto reef; frigatebirds and sooty terns by the thousand on Bush Key. Fort Jefferson is the largest brick masonry structure in the Americas, 16 million bricks, built from 1846 to guard the Gulf shipping lanes and never finished or fully armed. It held Dr Samuel Mudd, imprisoned for setting John Wilkes Booth's broken leg, who then saved the garrison during a yellow fever outbreak and was pardoned for it. Anchorage: One of the great American anchorages, but 70nm from Key West with no services, no fuel and limited shelter. Park permits required for overnight. Hazards: No site-specific hazards noted, brief normally. More