The fort was started in 1846, and, although work went on for almost 30 years, it was never finished. The U. S. Engineer Corps planned and supervised the building. Artisans imported from the North and slaves from Key West made up most of the labor gang. After 1861 the slaves were partly replaced by military prisoners, but slave labor did not end until Lincoln freed the slaves in 1863. _The War Between the States_ To prevent Florida’s seizure of the half-complete, unarmed defense, Federal troops hurriedly occupied Fort Jefferson (January 19, 1861), but aside from a few warning shots at Confederate privateers, there was no action. The average garrison numbered 500 men, and building quarters for them accounted for most of the wartime construction. Little important work was done after 1866, for the new rifled cannon had already made the fort obsolete. Further, the engineers found that the foundations rested not upon a solid coral reef, but upon sand and coral boulders washed up by the sea. The huge structure settled, and the walls began to crack. _Yellow Fever_ For almost 10 years after the war, Fort Jefferson remained a prison. Among the prisoners sent there in 1865 were the “Lincoln Conspirators”—Michael O’Loughlin, Samuel Arnold, Edward Spangler, and Dr. Samuel A. Mudd. Dr. Mudd, knowing nothing of President Lincoln’s assassination, had set the broken leg of the fugitive assassin, John Wilkes Booth. The innocent physician was convicted of conspiracy and sentenced to life imprisonment at hard labor. Normally, Tortugas was a healthful post, but in 1867 yellow fever came. From August 18 to November 14 the epidemic raged, striking 270 of the 300 men at the fort. Among the first of the 38 fatalities was the post surgeon, Maj. Joseph Sim Smith. Dr. Mudd, together with Dr. Daniel Whitehurst, from Key West, worked day and night to fight the scourge. Two years later, Dr. Mudd was pardoned. [Illustration: _It is a mile-long walk through the gunroom galleries._] _The Spanish-American War_ Because of hurricane damage and another fever outbreak, Fort Jefferson was abandoned in 1874. During the 1880’s, however, the United States began a naval building program, and Navy men looked at this southern outpost as a possible naval base. From Tortugas Harbor the battleship _Maine_ weighed anchor for Cuba, and when she was blown up in Havana Harbor, on February 15, 1898, the Navy began a coaling station outside the fort walls, bringing the total cost of the fortification to some 3½ million dollars. The big sheds were hardly completed before a hurricane smashed the loading rigs.