ra,It was started in 1846 and construction stopped around 1875. The Fort was never finished. Here are some facts.
Fort Jefferson is a massive but unfinished coastal fortress. It is the largest brick masonry structure in the Americas, and is composed of over 16 million bricks, the building alone occupying 16 acres. Among United States forts, only Fort Monroe in Virginia and Fort Adams in Rhode Island are larger. The fort is located on Garden Key in the lower Florida Keys within the Dry Tortugas National Park,