Fort Jefferson, Dry Tortugas National Park, FL, c. 1846 (via natgeo)

‘In the Gulf of Mexico, about 70 nautical miles west of Key West, Florida, a seven-mile long archipelago of seven low-lying islands forms the centerpiece of Dry Tortugas National Park. A bird and marine life sanctuary, it harbors some of the healthiest coral reefs remaining off North American shores. Towering incongruously in the midst of this subtropical Eden is Fort Jefferson, a relic of 19th-century military strategy.

‘Barely 93 acres of the park’s hundred square miles (64,000 acres) are above water. Three easterly keys are little more than spits of white coral sand. A stone’s throw from the visitor center in Fort Jefferson, Bush Key is home to a tangle of bay cedar, sea grape, mangrove, sea oats, and prickly pear cactus that reflects the original “desert island” character of the islands. The chain ends about three miles west with 49-acre Loggerhead Key, where a lighthouse completed in 1858 still flashes a beacon to mariners.

‘Spanish explorer Juan Ponce de León, the first European to describe the Florida peninsula, dropped anchor here in 1513. He found pellucid waters teeming with green, hawksbill, leatherback, and loggerhead turtles, and so named the islands las tortugas, which means “the turtles.” For the next three centuries, pirates relied on the turtles for meat and eggs; they also raided the sandy nests of roosting sooty and noddy terns, up to 80,000 of which descend on Bush Key every year between March and September. By 1825, when the islands’ first lighthouse began to alert sailors of surrounding reefs and shoals—a grave for more than 200 ships wrecked here since the 1600s—nautical charts warned that the Tortugas were “dry,” because of the lack of fresh water.

‘In 1846, U.S. Army strategists were concerned that hostile nations could disrupt shipping lanes in the Gulf of Mexico. As a result, they decided to build a 420-gun, 1,500-man fort on Garden Key. The intimidating bulk of the 45-foot-high, three-level hexagon, whose 2,000 archways run half a mile around, spared it from ever having to fire a shot in anger. A prison for Union deserters in the Civil War, it also held physician Samuel Mudd, who was convicted of conspiracy in Abraham Lincoln’s murder after he (unknowingly, he claimed) set the broken leg of fugitive assassin John Wilkes Booth. He served four years before being released.mStill unfinished after nearly 30 years of intermittent construction, the “Gibraltar of the Gulf” succumbed in 1874 to several factors: yellow fever, hurricane damage, and the new rifled cannon, which rendered its eight-foot-thick walls obsolete. Revived in 1898 as a Navy coaling station—the battleship Maine steamed from here to its infamous destiny in Havana’s harbor 90 miles south—the fort was permanently abandoned in 1907. In 1935 President Franklin D. Roosevelt signed the papers naming the site a national monument.’