It was to be a straightforward operation but, when the Poseidon was getting into position to fire a dummy torpedo at the HMS Marazion, something went wrong. As the Poseidon surfaced, she found herself in the path of a Chinese freighter, the SS Yuta.

The Yuta struck the hull of the Poseidon, carving it open. The order was given to abandon ship and those who could scrambled up the conning tower to escape, to be hauled aboard the freighter.

For those in the engine room, though, there was no time; they would have probably been drowned by the in-rushing water very quickly. However, having followed orders to close watertight doors, a group of eight men found themselves trapped in the forward torpedo compartment.

Before anyone could orientate themselves to their situation, the Poseidon gave a sickly lurch and plummeted 126 feet, to the seabed.

As the men picked themselves up from where they had been tossed in the descent, it became apparent what they were up against. In complete darkness and with water levels rising fast, they faced imminent death.

Among them were two unassuming officers' stewards: Ho Shung, a boy from Weihai, and Ah Hai, who would have been no older than 18.

The men found hope in their Davis Submerged Escape Apparatus (DSEA). The device, rudimentary by today's standards, contained a small cylinder of oxygen and a canister of barium hydroxide, which removes exhaled carbon dioxide.

Although the submariners aboard the Poseidon would have had some experience using it in compartment escape drills, those were carried out in a tank of water no more than 40 feet deep.

The younger steward, Ho, had no apparatus and Ah Hai, who had managed to grab a set, had never used it before. Not considered members of the Royal Navy, the stewards had never been given any escape training.

Not that it would have made much difference; in 1931, escape from a vessel far beneath the surface was purely conceptual - submarines were not equipped with escape hatches and the DSEA had never been used successfully in a submerged exit.

"Anyone who was a submariner in those days was a brave old devil," says Norman Cooke, who was a submarine-escape training instructor with the Royal Navy from 1963 to 1989. "They didn't have the equipment or the training and it would have been hellish down there."

Hellish was right. As the men accustomed themselves to their dire situation, it became clear they were going to have to attempt something that had never been done before - and not all of them would make it.

As the most senior ranking officer, Chief Petty Officer Patrick Henry Willis stepped up as leader, conferring with those who were most familiar with the torpedo compartment. They realised that in order to escape, water levels had to rise in the compartment to equalise the pressure.

This would allow them to open the forward torpedo loading hatch, which was their only means of escape. This process would take hours and they had limited oxygen in their DSEA and were already experiencing numbness in their limbs as hypothermia began to seize their exhausted bodies.

The men huddled together to hold onto the ladder up to the hatch. Aside from the sounds of spurting water and their own increasingly heavy breathing, it was silent. To conserve the power of their one electric torch it was only switched on intermittently, an ominous illumination in the pitch darkness.

Considering his age - details that emerged from his funeral in Weihai, which the Poseidon survivors attended, revealed he was no more than 15 - lack of DSEA and incomprehensible fear, it was no surprise that once when the torch was turned on, Ho was missing. He had succumbed to the dark water.

When Able Seaman George Hews complained of his oxygen being exhausted, Willis tried to assure him that his was the same and there was no need to worry. It did little to soothe Hews' panic and the next time the torch was switched on, he, too, had disappeared.

After a couple of hours, when it was thought the pressure had equalised enough, the first attempt was made. With a lurch, the hatch was opened just wide enough for Able Seaman E.G. Holt and Able Seaman A.J. Lovock to swim out before the pressure slammed it closed again, trapping the other four inside.

As Lovock exited the hatch, he struck his head on the casing. He would arrive at the surface dead, dragged by a barely living Holt.

Below the surface, the combination of increasingly toxic, compressed air, exhaustion and the onset of hypothermia was by the minute stealing away any chance of survival for the remaining men. Water levels needed to rise again to equalise pressure further before a second attempt to break free was made.

Hong Kong Maritime Museum's Stephen Davies describes the challenge: "Willis realised that with the quality of the air so dire, if they continued to go off in penny numbers, they wouldn't all make it - so the next attempt had to be everyone at once."

Finally, it was decided that the pressure in the compartment had equalised enough to attempt a second escape. One of the sailors, Able Seaman Vincent Nagle, had given Ah Hai a short, basic demonstration of how to use the DSEA equipment, but having never experienced compartmental escape and with a long way to the surface, it was unlikely the young man would survive the ascent.

On the surface, rescue and salvage vessels watched the water in silence. It had been an hour since Holt and Lovock had emerged. Then, to everyone's disbelief, four more men broke the surface - Ah Hai among them. This boy who had no training, no expertise and no experience in escape had survived.

"Of those that survived the ascent, Reginald Clarke and Ah Hai, whose conditions were viewed as most serious following their rescue, were treated with oxygen, both with two treatments of 20 minutes each, with a half-hour off the pure gas in between," says Steven Schwankert, a diving instructor and the author of .

"Today, this would be considered absolute minimum first aid for anyone believed to be suffering from DCS [decompression sickness, and would be] followed by treatment in a hyperbaric chamber."

It is likely that Ah Hai would have suffered the characteristic hunching and other effects of the bends for the rest of his life - a life we know almost nothing about.

The inventor of the DSEA, Robert Davis, sent a gift to each of the five survivors from the torpedo compartment: a silver cigarette case. In a gesture of gratitude and humility, Ah Hai replied on letter-headed paper from the HMS Medway ("Mieh Wei"), a ship that had taken part in the unsuccessful mission to salvage the Poseidon, enclosing a silver ashtray and model of a Chinese junk.

This letter, sent while the Medway was in Hong Kong, is the last we hear of Ah Hai.

"Mr Davis," the young man wrote. "This letter is for your great inspection. I received your letter yesterday and respectfully note all its contents. I thank you very much for the silver cigarette case which you have given me as a present. I am sending you by post your younger brother's [i.e. his own] photo, and beg you will accept this present favourably.

"I write this specially to inform you and offer you the New Year congratulations.

"(Your) younger brother Ah Hai, respectfully writing this. February 22nd, 1932. Sent from Mieh Wei ship, Hong Kong."

peacetime which results in the loss of one of the state-of-the-art submarines at the time - it's not the navy's finest hour, but that's how they portrayed it: 'Yes, we lost a ship, but look at the bravery of our men,'" says Schwankert.

The Royal Navy submariners who survived were all offered either a promotion or a pay rise. As for the wreck itself, Beijing admitted a few years ago that it had salvaged and taken apart the Poseidon, but claims no human remains were found within. What became of the bodies of the 18 sailors and Ho Shung remains a mystery.

The year following the disaster, 1932, saw the release of the feature film , which depicted the events surrounding the sinking of the Poseidon and heralded the survivors as courageous heroes. All except for Ah Hai, that is.

"There was no medal or audience with the king for him. Without further ado, Ah Hai seems to have gone back to work on ships," says Schwankert.

The afflictions of decompression sickness would have made this work difficult, not to mention the psychological impact the experience would have had. It is known that Willis suffered from severe anxiety later in life and it must be assumed the others would have, too.

Although reports in the at the time do not mention Ah Hai (recorded as "A Hee Hai") beyond listing him as a survivor, companies such as the Hongkong and Shanghai Bank, Jardine Matheson, the Hong Kong Jockey Club and Taikoo Sugar, and the staff of Dairy Farm, donated to a fund for the families of victims of the disaster.

The fund totalled nearly HK$40,000 within a week of the sinking, yet there is no mention of one of Hong Kong's own being among the "heroes".

Ah Hai would have been a distinctive character; a young Chinese man who had the bends. He would surely have been noticed - and the brave, modest man could still have family in Hong Kong. He may even be alive himself.

Poseidon: China's Secret Salvage of Britain's Lost Submarine (