HMS Poseidon was built by Vickers at Barrow-in-Furness and launched on 22 August 1929, a Parthian-class submarine designed for long-range patrols. She spent most of her brief career assigned to the Yellow Sea, operating from the Royal Navy's base at Weihai on the Shandong Peninsula -- a colonial outpost that Britain had leased from China in 1898 as part of the scramble for concessions that followed the First Sino-Japanese War. For Poseidon's crew, service in these waters meant months spent far from Britain, running exercises in the same seas where Chinese and Japanese fleets had clashed only decades before.

The collision with Yuta was sudden and devastating. Thirty-one crewmen scrambled into the water before the submarine sank, but others remained trapped inside the hull on the seabed. Poseidon carried the Davis Submerged Escape Apparatus, a closed-circuit breathing system that had entered service only two years earlier. It supplied pure oxygen and used a canvas drogue to slow the rate of ascent -- primitive technology by later standards, but groundbreaking at the time. The submarine lacked specialized escape compartments or flooding valves, so the men who attempted escape were improvising in conditions the apparatus had never been designed for. Eight crewmen managed to leave through the forward end of the boat. Two never reached the surface, and one died after being rescued. But the fact that six men survived an unaided escape from 130 feet was remarkable enough to transform Admiralty policy.

Before Poseidon, the Royal Navy's standing advice to crews trapped in a sunken submarine was to wait for rescue -- sit tight, conserve oxygen, and hope that divers or salvage vessels would arrive in time. The successful escapes from Poseidon proved this doctrine fatally conservative. If men could reach the surface on their own, waiting for help that might never come was the greater risk. In March 1934, the Admiralty formally reversed its guidance, announcing in the House of Commons that submariners should attempt escape as soon as possible rather than waiting passively. The change saved lives in submarine disasters for decades to come. Twenty-one of Poseidon's crew died in total -- a heavy price, but one that purchased a fundamental shift in how navies prepared their submarine crews for the worst.

For forty years, HMS Poseidon lay undisturbed on the seabed north of Weihai. Then, in 1972, China's newly formed underwater recovery units quietly raised the submarine. The operation remained unknown in the West for three decades, until researcher Steven Schwankert discovered an article about the salvage in the Chinese magazine Modern Ships during a Google search and confirmed the details at a Hong Kong library. The discovery raised uncomfortable questions. In the former British naval cemetery on Liugong Island, historians found the gravestones of Poseidon's lost sailors stacked in haphazard piles, bearing clearly legible names, dates, and epitaphs. The British government asked Beijing for an explanation.

Schwankert's investigation became the book Poseidon: China's Secret Salvage of Britain's Lost Submarine and a documentary film, The Poseidon Project. The story captures the overlapping histories that haunt this stretch of the Shandong coast -- a British submarine sunk in waters that Britain controlled as a colonial concession, resting near an island that had served as headquarters for a Chinese fleet destroyed by Japan, eventually raised by a Communist government that had no diplomatic relations with Britain at the time. The waters off Weihai hold these layers of history in suspension: the graves of British submariners scattered by a salvage crew that may not have known what they were disturbing, the wreck of a boat that proved men could escape from the deep, the quiet seabed that keeps returning its secrets when you least expect it.