Dry Tortugas National Park, Key West

This national park is the nation's only at-sea national park and is home to the historic Fort Jefferson. Located about 70 miles west of Key West and accessible only by boat or chartered seaplane, the park and fort lure divers, snorkelers and nature lovers who flock here to get a close look at stag horn coral, angel fish, rare birds and the loggerhead turtles that gave the island its name. Built in 1846, the five-sided fort covers more than 11 of the island's 16 acres.