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The story of the British submarine that mysteriously sank in waters off China in 1931, the sailors who survived, and their descendants, in search of the truth about what happened.


Beijing-based journalist and part-time scuba instructor Steven Schwankert was looking for interesting wrecks to dive in northern China when he found HMS Poseidon on a list of unexplored dive sites. His private obsession with bringing wreck diving to China became a public and personally high-risk mission to uncover a secret history of political intrigue that took him half way across the world in search of evidence.

On June the 9th 1931, one of Britain’s most advanced submarines collided with a cargo ship off the coast of China. Half of the crew escaped in the first few seconds, but the rest of the men sank 40 metres down to the bottom of the sea, entombed in their boat. Three hours later, six sailors surfaced, barely conscious and bitterly cold, as their comrades watched on in amazement. They were the first men ever to escape alive from a sunken submarine using a proto- scuba device.

Their story hit newspaper headlines worldwide and went on to inspire a feature film, released later that year. Their miraculous escape changed marine safety forever, leading to a revolution in submarine design and a big leap forward in our understanding of decompression sickness. But their names, and their submarine, gradually sank into obscurity.

Steven Schwankert’s six-year search for the submarine revealed a story that was far more complicated than he had first expected, challenging official accounts of the escape, and uncovering the truth behind the forgotten wreck.

“A very impressive and compelling narrative.”
George Malcolmson, Royal Navy Submarine Museum

“Loved the film. Absolutely loved it. Crisp, tight, informative, dramatic.”
Julian Ryall, The Telegraph

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