Ft Jefferson is an old fort, and then a prison. The only way to get there is sea plane or high speed boat for a couple hours. It is the largest brick structure.... It is a wonderful place! Almost 70 miles (112.9 km) west of Key West lies a cluster of seven islands, composed of coral reefs and sand, called the Dry Tortugas. Along with the surrounding shoals and waters, they make up Dry Tortugas National Park. The area is known for its famous bird and marine life, its legends of pirates and sunken gold, and its military past.