In 1931, Britain's most advanced submarine collided with a cargo ship off the coast of China and sank. Three hours later, six sailors surfaced, barely conscious. They were the first men ever to escape from a sunken submarine using a proto-scuba device. Their story hit headlines and went on to inspire a feature film. The miraculous escape changed marine safety forever. But their names, and their submarine, gradually sank into obscurity. Beijing-based journalist and scuba instructor Steven Schwankert was looking for nearby wrecks to dive when he found HMS Poseidon on a list of unexplored sites. His five-year search for the submarine became a private obsession that would challenge official accounts of the escape, and reveal a secret history.