HMS Sidon (P259) was a British S-Class Submarine built in 1944.
16th June 1955: While moored and loading an experimental torpedo in Portland Harbour a massive explosion blew out the watertight bulkheads at the forward end of the submarine. 12 submariners died in the explosion, and a medical officer died as the submarine settled onto the bottom of the harbour in less than 20 minutes. The explosion was due to the running of the torpedo motor within the submarine and a rupture of a peroxide pipe that then come into contact with copper fittings releasing Oxygen - the warhead did not explode but the gases ignited, sending large pieces of torpedo through the submarine with enormous force. The Sidon was raised from the harbour on the 22nd June.
She was finally sunk upright on a gravel seabed as an ASDIC (SONAR) target in 19 June 1957.
Location: 50 32'97" N 02 38'48" W
Access: Hard Boat