HMS Thetis was a Group 1 T-class submarine of the Royal Navy which served under two names. Under her first identity, HMS Thetis, she commenced sea trials on 4 March 1939. She sank during trials on 1 June 1939 with the loss of 99 lives. She was salvaged, repaired and recommissioned as HMS Thunderbolt serving in the Atlantic and Mediterranean theatres until she was lost with all hands on 14 March 1943. This makes Thetis one of the few military vessels that have been lost twice with her crew in their service history, like the H. L. Hunley.
Thetis was built by Cammell Laird in Birkenhead, England and launched on 29 June 1938. After completion, trials were delayed because the forward hydroplanes jammed, but eventually started in Liverpool Bay under Lieutenant-Commander Guy Bolus. Thetis left Birkenhead for Liverpool Bay to conduct her final diving trials, accompanied by the tug Grebecock. As well as her normal complement of 59 men she was carrying technical observers from Cammell Laird and other naval personnel, a total of 103 men. The first dive was attempted on 1 June 1939 at about 14:00. The submarine was too light to dive, so a survey of the water in the various tanks on board was made. One of the checks was whether the internal torpedo tubes were flooded.
Lieutenant Frederick Woods, the torpedo officer, opened the test cocks on the tubes. Unfortunately, the test cock on tube number 5 was blocked by some enamel paint so no water flowed out even though the bow cap was open. Prickers to clear the test cocks had been provided but they were not used. This combined with a confusing layout of the bow cap indicators - they were arranged in a vertical line with 5 at the bottom - 1,2,3,4,6, and then 5; and the shut position for tube 5 on the dial was in a different position from those of the other torpedo tubes - led to the inner door of the tube being opened. The inrush of water caused the bow of the submarine to sink to the seabed 150 ft (46 m) below the surface.
An indicator buoy was released and smoke candle fired. By 1600, Grebecock was becoming concerned for the safety of Thetis and radioed HMS Dolphin submarine base at Gosport. A search was immediately instigated. Although the stern remained on the surface, only four crew escaped before the rest were overcome by carbon dioxide poisoning caused by the crowded conditions, the increased atmospheric pressure and a delay of 20 hours before the evacuation started. Ninety-nine lives were lost in the incident. In addition to the normal crew of 53, there were 26 Cammell Laird employees, another 9 naval officers, 4 Vickers-Armstrong employees and 2 caterers. The crew waited before abandoning the vessel until it had been discovered by Brazen, a destroyer which had been sent to search for it and which indicated her presence by dropping small explosive charges into the water.
The incident attracted legal action from one of the widows, who brought a claim of negligence against the shipbuilders, for not removing the material blocking the valve. Unfortunately for her the Admiralty successfully invoked Crown Privilege (now termed Public Interest Immunity) and blocked the disclosure of, amongst other items, 'the contract for the hull and machinery of Thetis' as evidence in court, on the basis that to do so would be 'injurious to the public interest'. The case is one of interest in English law, as the judges in this case accepted the Admiralty's claim on face value with no scrutiny, a ruling later overturned.
One further fatality occurred during salvage operations, when Diver Petty Officer Henry Otho Perdue died from "the bends" on 23 August 1939. On Sunday 3 September Thetis was intentionally grounded ashore at Red Wharf Bay, Anglesey. It was the same day that war was declared. Human remains that had not already been removed by the salvage team were now brought out to a Naval funeral, with full honours.
The Thetis disaster was in marked contrast to the successful rescue of the survivors of USS Squalus, which had sunk off the coast of New Hampshire just a week previously.