Explore the Dry Tortugas and Fort Jefferson.
Fort Jefferson is the largest brick masonry structure in the Americas, and is composed of over 16 million bricks.
The fort itself is around 47 square acres total and has been around since the Civil War. It was used as a prison base in the Civil War, as well as a defense to the Florida coastline.
During the Cold War, a nuclear silo was planned to be built at Fort Jefferson, but the idea was scrapped due to the difficulty of managing a silo on such a small island that far from the coast.