HMS Poseidon (P99) was a Parthian-class submarine designed and built by Vickers Shipbuilding and Engineering in Barrow-in-Furness, England for the Royal Navy, launched on 22 August 1929.

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.

[6][7] 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.

[7][8] 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.