HMS Poseidon (P99)  HMS "Poseidon" (P99) was a designed and built by Vickers Shipbuilding and Engineering in Barrow-in-Furness for the Royal Navy, launched on 22 August 1929. She spent most of her short career assigned to the Yellow Sea region, based at the Royal Navy's base at Weihai, China. In 1931, the submarine sank after a collision with the steamship "Yuta" north of Weihai. The submarine was later secretly salvaged by the Chinese in 1972. At about 12:12 on 9 June 1931, while exercising on the surface with the submarine tender north of the vessels' base at Weihai, and despite excellent visibility, "Poseidon" collided with the Chinese merchant vessel SS "Yuta". Thirty-one of the submarine's crew managed to scramble into the water before the submarine sank to the seabed below within a few minutes. The aircraft carrier , heavy cruiser and sister submarine led the rescue operations. "Poseidon" was equipped with Davis Submerged Escape Apparatus which had come into service two years earlier. This was a closed circuit underwater breathing system which provided the wearer with a supply of pure oxygen and a canvas drogue to slow the rate of ascent. Despite the submarine not being equipped with specialised escape compartments or flooding valves, eight of the crew managed to leave the forward end of the boat, although two failed to reach the surface and one died later. Twenty-one crew died in total. A consequence of the successful escape of part of the crew was to change Admiralty policy from advising crews to wait for the arrival of assistance to attempting to escape from the submarine as soon as possible. This policy was announced in the House of Commons in March 1934. The secret salvaging of the submarine in 1972 by China's then newly formed underwater recovery units was described in 2002 in an article in the popular Chinese magazine "Modern Ships" (). This was not known about in the West until the researcher and journalist Steven Schwankert discovered that article with a Google web search and later read it in a Hong Kong library. In the former British naval cemetery on the island of Liugong, gravestones, bearing clearly legible names, dates and epitaphs of the lost sailors were found in haphazard stacks by historians looking into the sinking of HMS "Poseidon" and its salvage by the Chinese. The British Ministry of Defence has not received an answer to what became of the remains of the crew.