R-12 was an R-1-class coastal defense submarine, built at the Fore River Shipbuilding Company in Quincy, Massachusetts. Her keel was laid down on March 28, 1918, designed for a war that ended before she could fight it. She launched on August 15, 1919, and was commissioned at the Boston Navy Yard on September 23 of that year. For the next decade she served across the Pacific -- Pearl Harbor, the California coast, Johnston Island -- the routine life of a peacetime submarine. In December 1930, she and eighteen sister boats made the long voyage back to the East Coast. By 1932, she was decommissioned and mothballed at the Philadelphia Navy Yard, seemingly finished. But when the United States began building a two-ocean navy in 1940, trained submarine crews were desperately needed. The old R-class boats were pulled from reserve to serve as training vessels. R-12 was recommissioned on July 1, 1940, and sent south.

R-12 spent the early war years shuttling between training stations across the Caribbean and the East Coast. She operated out of Coco Solo in the Panama Canal Zone, ran exercises in the waters around the Perlas Islands, trained crews at Guantanamo Bay, and worked out of Key West. She also operated from New London, Connecticut -- the Navy's submarine school -- and patrolled off Provincetown and Casco Bay in the anxious days after Pearl Harbor. Hers was not glamorous duty. While newer fleet submarines hunted Japanese shipping in the Pacific, the old R-boats drilled the men who would crew those boats. R-12 was rated a "well-organized and well-trained submarine" by her inspectors. On the morning of June 12, 1943, she put to sea from Key West with 47 people aboard for routine torpedo practice.

After conducting sound exercises with the coastal yacht Coral, R-12 surfaced and began preparing for a torpedo firing run. She shifted from electric motors to diesel engines and proceeded on course 275 degrees, rigged for dive. Then the flooding started. The forward battery compartment filled faster than anyone could react. The collision alarm sounded from below. Even with the ballast tanks being blown, the submarine's bow dipped and the boat slid forward into the sea. The five men on the bridge had no time to close hatches or reach the interior -- they were simply washed off as R-12 disappeared beneath them. A fourteen-day search involving as many as fourteen ships located the wreck on June 23, 1943, sitting in roughly 600 feet of water southeast of Key West. She was stricken from the Navy Register on July 6. The Navy's Court of Inquiry concluded that rapid flooding through a forward torpedo tube was the probable cause, but the official reason for the loss was never determined.

R-12 sat on the bottom, largely forgotten, for nearly seventy years. Sometime before May 2011, an exploration team led by Tim Taylor aboard the research vessel Tiburon located and documented the wreck using an autonomous underwater robot. The imagery they captured was the first ever taken of R-12's remains. Taylor's team shared their findings with the U.S. Navy and planned future expeditions to investigate the cause of the sinking more closely. The discovery brought a measure of closure, but the fundamental mystery endures: what exactly caused the forward battery compartment to flood so catastrophically that a submarine sank in fifteen seconds? The old boat took her secret with her.

The submarine community remembers its lost boats with a phrase: "on eternal patrol." R-12 is honored with granite markers at the National Submarine Memorial in Groton, Connecticut, and at the Pacific Fleet Submarine Memorial Museum in Pearl Harbor, Hawaii. A small monument stands at the Rhode Island Veterans Cemetery in Exeter. These memorials mark a loss that happened not in combat but in the mundane work of training -- a reminder that submarines were dangerous even when the enemy was nowhere near. The 42 men who died aboard R-12 included career sailors, young trainees learning the submarine trade, and two Brazilian naval observers who had come to watch American methods. They are all still down there, southeast of Key West, in 600 feet of warm Caribbean water.