Dry Tortugas National Park, located 70 miles west of Key West, is most famous for Fort Jefferson, the largest 19th-century coastal brick fortress in the United States. Composed of over 16 million bricks, the massive hexagonal fort was built to protect the strategic deep-water anchorage of the Gulf of Mexico, though it was never fully completed. Beyond the fort, the park is world-renowned for its vibrant coral reefs and crystal-clear turquoise waters, offering some of the best snorkeling and diving in the Florida Keys. Because of its extreme isolation, it is a "Dark Sky" sanctuary and a critical nesting ground for thousands of Sooty Terns and Frigatebirds. In 2026, it is a bucket-list destination because it is only accessible by seaplane or the "Yankee Freedom" ferry, ensuring the crowds remain small. It also holds a dark place in American history: it served as a military prison during the Civil War, famously housing Dr. Samuel Mudd, the physician who treated John Wilkes Booth after the assassination of Abraham Lincoln. For a 2026 visitor, it represents the ultimate "off-the-grid" experience, combining American military history with untouched tropical marine biodiversity.