HMS THETIS (Lt Cdr G H Bolus) sailed from Birkenhead on 1 June 1939 for initial trials in Liverpool Bay after building. Her standard complement was 53 but she had an additional 50 people on board made up of four (4) officers from other submarines building, five (5) other naval officers, seven (7) Admiralty personnel, twenty-six (26) Cammell Laird employees, four (4) Vickers employees, one (1) Browns Bros employee, one (1) Mersey Pilot and two (2) City Caterers staff. Amongst the additional naval officers on board was captain of the squadron that THETIS was due to join, Captain Oram, who ten years before had been in command of L12 when she had collided with H47.

Diving proved problematic: THETIS was too light. Her internal compensating tanks were all filled and orders were given to check the state of her torpedo tubes. These were normally checked by opening a small test cock in each tube rear door to see if any water emerged. If no water emerged, it was deemed safe to open the rear door.

The first 4 tubes checked empty. On opening the test cock on Tube 5, it appeared that the tube was dry and steps were taken to open the rear door. As the locking mechanism was disengaged, the rear door was flung open by the pressure of water in the tube and, despite the efforts of the torpedo crew, the compartment flooded rapidly. The submarine soon sank and settled with her bow on the bottom and her stern out of the water.

Efforts were made to enter the fore ends through the escape hatch, but these proved unsuccessful. The accompanying tender signalled that something was amiss, and a search operation was initiated although this was confounded by some errors in positional reporting.

Onboard the submarine, 4 men managed to escape through the after escape tower (which was below the water). They reported that conditions on board were deteriorating rapidly (as twice the normal number of men were on board). Rescue vessels eventually arrived on the scene and attempts were made to secure the after end of the submarine and cut a hole in the pressure hull to allow the men inside to escape. Unfortunately, these proved ineffective, and the submarine sank. Ninety-nine men died.

THETIS was subsequently recovered and re-commissioned as HMS THUNDERBOLT (q.v.).