HMS Thetis (N25) was a Group 1 T-class submarine of the Royal Navy which sank during sea trials in Liverpool Bay, England on 1 June 1939.

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 Grebe Cock.

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.

[4] Although the stern remained on the surface, only three RN personnel (Lieutenant Frederick Woods, Captain Harry Oram and Leading Stoker Walter Arnold) and one Cammell Laird man (Fitter Frank Shaw) 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.

[3] The crew waited before abandoning the vessel until she had been discovered by the destroyer Brazen, which had been sent to search for her and which indicated her presence by dropping small explosive charges into the water.

Four men, three RN personnel (Lieutenant Woods, Captain Oram and Leading Stoker Arnold) and one Cammell Laird's employee (Fitter Shaw) successfully used the escape chamber.

[5] 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.

[10] 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.

Thunderbolt's objective was shipping in Cagliari, but the operation was not a success, and P311 was lost at La Maddalena, her intended target.

On 2–3 January, the manned torpedoes entered the harbour and mined the ships there, sinking the hull of the incomplete light cruiser Ulpio Traiano and the freighter SS Viminale.

The British submarine was forced to dive and escape by the combined fire of the Italian auxiliary cruiser Brindisi and a coastal battery, while the sailboat only received light damage.

The cause of the loss of Thetis – flooding due to both inner and outer torpedo hatches being open to the sea – was used in the 1968 film Ice Station Zebra, where the character played by Patrick McGoohan describes a method of sabotaging a submarine by blocking the tube test cocks, fooling a torpedoman into believing the outer hatch was closed.

Alexander Fullerton's 1994 novel Not Thinking of Death centres around a fictionalised account of the sinking (with Thetis renamed to Trumpeter).

In 1999, a play entitled HMS Thetis by Mark Gee in association with David Roberts, was performed at the Liverpool Bluecoat Chambers and at Birkenhead's Pacific Road Theatre.

The play starred John McArdle and also the newly employed First Year Apprentices from Cammell Laird Shipyard (Paul Gillies, Dave Gill, Alan Lane, Chris Motley, Mike Jebb, Steve Taylor, Ollie Dodson, Stuie Dicken, Mark Poland, Ben McDonald, Tony Cummins, Barry Hayes, Chris Hall, Martin King, Graham Crilly, Billy Coburn, Matty Brassey).

It covered the loss of the vessel and the subsequent enquiry, together with interviews with relatives of two of the men lost in the tragedy and the son of a survivor, Leading Stoker Arnold.