Seventy miles west of Key West, a group of islands forms the Dry Tortugas. Those islands, and the waters surrounding them, are at the center of a national park with spectacular coral reefs. But the park is best known for its biggest structure. Fort Jefferson was part of a system of coastal fortifications built in the 19th century — and after 30 years of work, it was never finished. The fort had more than 400 guns and a standing garrison of soldiers. But it never engaged in battle.