Construction on the "Gibraltar of the Gulf," Fort Jefferson, began in 1846 at the western end of the Florida Keys to protect the Florida Straits. The largest all-masonry fort in the Western Hemisphere, construction on the fort continued for 30 years until after the advent of rifled cannon made the 8-foot thick walls obsolete. With the threat of Florida's secession from the Union in late 1860, the superintending engineer for construction at Fort Jefferson, Captain Montgomery C. Meigs, called for reinforcements to prevent its seizure by Southern militia. In response, the gunboat USS Mohawk was sent from Key West in November 1860 to discourage any seizure attempts. In January 1861, after Florida had seceded from the Union, a force of 66 men of the 2nd U.S. Artillery arrived at Fort Jefferson from Boston, and the fort was secured for the Union. In May 1861, Meigs was promoted to brigadier general and served as the Quartermaster General of the Union Army during the Civil War. For the remainder of the Civil War, Fort Jefferson served as an important Union military post and a military prison. At one point, it was home to some 2,000 occupants, including soldiers, their families, laborers, and prisoners. In July 1865, Fort Jefferson received its most infamous prisoners when four men convicted for conspiracy in the assassination of President Lincoln were imprisoned there, including Dr. Samuel Mudd, the Maryland physician who had set the broken leg of John Wilkes Booth. During a yellow fever outbreak in 1867, after the prison doctor died, Dr. Mudd risked his life to provide treatment to his jailers and the soldiers of the fort. Due largely in part to this life-saving service, President Andrew Johnson pardoned Mudd in 1869. As one of the most remote parks in the national parks system, Fort Jefferson is accessible only by boat and seaplane which depart from Key West, 70 miles away.