HMS SIDON (Lt H T Verry) was outboard the Submarine Depot Ship, HMS MAIDSTONE, at Portland on 16 June 1955 preparing for firings of experimental torpedoes. No warheads were fitted to these torpedoes, but they did contain experimental fuel comprising highly volatile hydrogen peroxide.
The crew had just embarked the torpedoes before going to sea for trial firings. Without warning one of the experimental weapons loaded in a torpedo tube exploded, seriously damaging the submarine, and killing and injuring many of the crew. Lt Verry and others from MAIDSTONE formed a rescue party.
At 0845 the submarine sank unexpectedly. Of the 56 men onboard three officers and ten ratings died but the remainder survived. The wreck was raised on 23 June 1955 and the casualties were recovered two days later. Beyond repair, the intact wreck was sunk again a few miles West of Portland and used as a target.