"Fort Jefferson, the largest all-masonry fort in the United States, was built between 1846 and 1875 to protect the nation's gateway to the Gulf of Mexico. Supply and subsidence problems and the Civil War delayed construction. The fort was never completed because of fears that additional bricks and cannon would cause further settling and place more stress on the structure and the cistern system. Distinguishing features include decorative brickwork and 2,000 arches. Time, weather, and water continue to take their toll, necessitating ongoing stabilization and restoration projects."