Almost 70 miles (113 km) west of Key West, Dry Tortugas National Park is 100 square miles (nearly 260,000 sq. km) of mostly open water with seven small islands. Accessible only by private board, daily ferry or seaplane, it is rich with marine and bird life and history. The six-sided Fort Jefferson, one of the largest U.S. military forts ever built, was constructed there in the mid-19th century to protect ship traffic and used as a prison. Dr. Samuel Mudd, the physician to treated Abraham Lincoln's assassin, served four years of life sentence for treason there before being pardoned. The ferry trip takes about two hours one-way with same-day return or an option of overnight camping.