HMS Thetis sank in 1939 after becoming trapped on the seabed during sea trials.
The T-class submarine went down in Liverpool Bay on 1 June 1939 while undergoing acceptance trials before commissioning. A torpedo-tube test created confusion because a bow cap indicator suggested that a tube was empty when it still contained a torpedo. When the outer door was opened, seawater entered the forward compartment.
Thetis settled on the seabed with its bow above the surface. Rescue efforts were severely hampered by flooding, limited escape equipment and difficult access. Ninety-nine people died, while four escaped through the submarine’s escape arrangements.
The submarine was later salvaged and recommissioned as HMS Thunderbolt. That name can cause confusion in historical accounts: Thetis refers to the original disaster, while Thunderbolt refers to the rebuilt boat’s later wartime service.