Encompassing over 100sq mi in the Gulf of Mexico, the Dry Tortugas National Park protects the small cluster of reef islands known as the Dry Tortugas. One of these, christened Garden Key, is the site of Fort Jefferson, the largest coastal stronghold built by the US in the 19C. You can climb the battlements to an abandoned lighthouse and enjoy the spectacular view. The surrounding waters are perfect for diving, fishing and the exploring underwater wrecks.