The submarine, HMS Thetis, was lost in 1939, a few weeks before the start of the Second World War. On 1st June she sailed from her builder's yard at Cammel Laird, Birkenhead into Liverpool Bay on her first full sea trial. There were 103 men on board, 50 more than the normal complement of crew. Included were a group of Naval observers, a large number of employees of the shipbuilders, some caterers and a Mersey Pilot. On her first test dive there was a mishap with the forward No. 5 torpedo tube resulting in the first two compartments flooding and her bow being firmly stuck in the seabed mud. Only 4 men managed to escape. The other 99 died, trapped in the submarine. The vessel was eventually raised and bodies recovered, with 44 of these buried in a mass grave at the Maeshyfryd Cemetery at Holyhead.

In tribute to those lost the Maritime Museum has a special exhibit illustrating this tragic story, which contains many interesting artifacts and photographs.