When the British submarine HMS Poseidon sank off China's east coast 82 years ago after colliding with a cargo ship, the dramatic underwater escape by five of its crew members made headlines around the world.

But the episode was soon overshadowed by the communist insurgency already raging on the mainland, the Japanese invasion of Manchuria, and eventually the outbreak of the second world war. The world moved on, the wreck of the Poseidon lay 30 metres beneath the sea, lost to history.

Until now. A new book reveals that China secretly salvaged the submarine in 1972, perhaps to abet its then-incipient nuclear submarine programme.

Steven Schwankert, an American author and diving-company owner in Beijing, spent six years obsessively piecing together the submarine's story; his book about the experience, Poseidon: China's Secret Salvage of Britain's Lost Submarine, was released this month....