Thetis carried her normal crew of 59 officers and men accompanied by another 44 shipyard workers, technical advisors, and naval observers when she attempted to dive on June 1, 1939. The crew, unable to determine that the torpedo tube outer doors were open because the drain tubes were clogged with paint, opened the internal doors of at least one tube. Efforts by the inexperienced crew to correct the problem were complicated by overcrowded conditions, leading to uncontrolled flooding in the forward torpedo room.
Emergency surfacing procedures were initiated and Thetis raised its stern to the surface before heeling to port and beginning a dive. Thetis sank bow down in 150 feet of water. Most of the crew survived the initial sinking and after signaling to accompanying observation ships they prepared to abandon the submarine using prearranged escape procedures.
The submarine was equipped with an escape hatch which required the user (only one man could escape at a time) to follow procedures exactly or the system would be disabled for those following. After the first four men cleared the escape hatch one by one, the fifth escapee panicked as he attempted the procedure, disabling the escape mechanism and dooming the rest of the crew to a slow death from carbon dioxide poisoning.
Thetis was over 275 feet long, and a goodly portion of its hull remained above the surface of the water, tantalizingly exposed to rescuers who could do little to help the men aboard. The option of cutting air holes in the exposed stern of Thetis was considered and rejected as being too dangerous to the integrity of the hull and the safety of the would-be rescuers. Over the course of the next 48 hours, the crew aboard Thetis continued to breathe.
Ninety-nine men died as the natural result of their own exhalations. The Thetis disaster remains Britain’s largest loss of life in a submarine.
While Thetis remained helplessly crippled and the families of its lost mourned, the US Navy succeeded in raising USS Squalus (after a dramatic rescue, broadcast live on radio, in which 33 survivors of the sinking were saved). Squalus was raised in September 1939, returned to the shipyard, and eventually recommissioned as USS Sailfish. Under that name, it served throughout World War II, and was credited with sinking 20 Japanese ships for a total of more than 80,000 tons. Throughout its career it was forbidden for anyone of its crew, on pain of being marooned at the next port of call, to utter the word Squalus.
Efforts to salvage Thetis began as Squalus was being raised. At least one diver died while preparing to raise Thetis. The effects and dangers of narcosis – called “the bends” by divers – were as yet poorly understood, leading to the loss of Petty Officer Henry Perdue as preparations were made to bring Thetis back to the surface. By early September Thetis was raised off the bottom and – still submerged – deliberately grounded in Anglesey, on the Welsh coast. With England now at war with Germany efforts to prepare the submarine for service intensified.