Fort Jefferson: The Unfinished Fort in Open Ocean, Florida

In 1513, Juan Ponce de León reached a cluster of low islands west of the Florida Keys, caught 160 sea turtles, and named them the Tortugas. Without fresh water, sailors later added a warning: the Dry Tortugas. Three centuries later, the United States wanted to control the shipping lanes between the Gulf of Mexico and the Atlantic. In December 1846, construction began on Garden Key on a fort meant to guard one of the best anchorages in the region. The work went on for nearly 30 years. More than 16 million bricks went into the walls, making it what Wikipedia calls the largest brick masonry structure in the Americas. The fort covers 16 acres, and its plans called for 450 guns on three levels. By 1872, it held 243 large guns. None of them was ever fired in battle. During the Civil War, the fort stayed in Union hands and became a prison, mostly for Union deserters. In 1865, four men convicted in the Lincoln assassination conspiracy were sent there, including Dr. Samuel Mudd. During the 1867 yellow fever epidemic, Mudd treated the sick. He was later pardoned. Hurricanes and disease wore the fort down. The Army withdrew its garrison in 1874, and in 1889 the fort was handed over for use as a quarantine station. In 1935, President Franklin D. Roosevelt made it a national monument, and in 1992 it became the centerpiece of Dry Tortugas National Park. The fort was never finished, and the park around it is more than 99% water.