The sinking of HMS Thetis on June 1, 1939, remains the deadliest peacetime submarine disaster in British naval history, claiming the lives of 99 out of the 103 men on board. The newly constructed vessel was undergoing a test dive in Liverpool Bay after being completed by Cammel Laird shipbuilders in Birkenhead.

The submarine carried 53 crew members alongside 50 additional personnel, including officers from other submarines, Admiralty staff supervising the test, shipbuilders, and catering personnel. After departing for the trial run, the crew prepared for the submarine’s first dive shortly after lunch. However, the boat appeared reluctant to submerge, prompting the captain to inspect the six bow torpedo tubes.

The test protocol required flooding two tubes to simulate the weight of torpedoes, but confusion arose during this check. When test cocks were opened, water flowed from tube number six as expected, but not from tube five. Investigations later revealed that a hole inside the tube’s door had been inadvertently painted over, blocking water flow. Additionally, the water-level indicator was either improperly wired or positioned in a way that led to misinterpretation.

Believing tube five was empty, crew members opened it to inspect within. In reality, water was present, causing an uncontrollable influx that prevented closing the door. The compartment had to be abandoned as flooding spread, and electrical power failed in that area. Compounding the situation, the adjacent compartment’s door could not be sealed, resulting in two flooded sections and rapid descent to the seabed.

The submarine did not surface after its scheduled three-hour dive, prompting the escorting tug to report the delay via radio. A telegram relay to naval command followed, but by then, over three and a half hours had passed with the submarine stranded underwater. Rescue ships were dispatched but arrived late in the evening and searched in an incorrect area, unaware the vessel lay roughly four miles from the reported position.

Inside the submarine, the crew attempted to lighten the boat by emptying drinking water and fuel tanks, although the flooded forward compartments could not be addressed. These efforts raised the stern about 18 feet above the surface, allowing part of the vessel to break through the water. The next morning, a destroyer spotted the exposed stern, but by then, the submarine had been submerged for more than 19 hours. Air quality deteriorated rapidly due to the confined space and overcapacity, with 103 people on board a vessel designed for far fewer.

With the stern emerging, the rear escape hatch was near the surface. Two men successfully escaped initially. When four more men attempted simultaneous egress, one panicked as the airlock filled with water, blocking the outer hatch from opening. Three men drowned inside the airlock; two others managed to escape. Rescue efforts continued with a steel cable from a nearby ship attached to the submarine to keep the stern above water, but the cable snapped, and the stern submerged again.

HMS Thetis was eventually raised, repaired, and recommissioned as HMS Thunderbolt. The submarine served successfully during World War II until it was sunk a second time in March 1943, with the loss of all personnel on board.