Dry Tortugas

Fort Jefferson, the centerpiece of Dry Tortugas National Park, is the largest masonry structure in the Western Hemisphere, built from over 16 million bricks on a tiny island roughly 70 miles from the nearest paved road. Construction began in 1846 and never fully finished, a coastal fortress rendered strategically obsolete by advances in weaponry before its guns were ever fired in battle, though it did serve as a Civil War prison, most famously holding Dr. Samuel Mudd, convicted of aiding Abraham Lincoln's assassin.

The park itself is made up of seven small islands, and getting there requires real commitment: a roughly two-and-a-half-hour high-speed ferry ride or a shorter but pricier seaplane flight from Key West, since there's no other way in. Once there, the reward is turquoise water, some of the best snorkeling in the Florida Keys right off the fort's walls, and a genuine sense of isolation that's increasingly rare this close to a major tourist destination.

Come for the fort. Stay for the water around it. Read the story, book your ferry or seaplane well ahead, and when you leave, collect the stamp.