Image caption HMS Perseus was launched in May 1929

Seventy years ago, off the Greek island of Kefalonia, the British submarine HMS Perseus hit an Italian mine, sparking one of the greatest and most controversial survival stories of World War II.

The clear waters of the Mediterranean were a death trap for British submarines in World War II.

Some were bombed from the air, others hunted with sonar and depth charges, and many, perhaps most, collided with mines.

Two fifths of the subs that ventured into the Mediterranean were sunk and when a submarine sank it became a communal coffin - everyone on board died. That was the rule.

In fact, during the whole of the war there were only four escapes from stricken British submarines. And the most remarkable of these took place on 6 December 1941, when HMS Perseus plummeted to the seabed.

Enigma

When she left the British submarine base at Malta at the end of November 1941, HMS Perseus had on board her 59 crew and two passengers, one of whom was John Capes, a 31-year-old Navy stoker en route to Alexandria.

Image caption John Capes: Stoker on the Perseus

Tall, dark, handsome and a bit of an enigma, Capes had been educated at Dulwich College, and as the son of a diplomat he would naturally have been officer class rather than one of the lowliest of the mechanics who looked after the engines.

On the rough winter night of 6 December, Perseus was on the surface of the sea 3km (two miles) off the coast of Kefalonia, recharging her batteries under cover of darkness in preparation for another day underwater.

According to newspaper articles Capes later wrote or contributed to, he was relaxing in a makeshift bunk converted from a spare torpedo tube when, with no warning, there was a devastating explosion.

The boat twisted, plunged, and hit the bottom with what Capes described as a "nerve-shattering jolt".

His bunk reared up and threw him across the compartment. The lights went out.

Escape from the Deep Louis de Bernieres returns to Kefalonia to tell the story of John Capes and HMS Perseus

Tim Clayton acted as a programme consultant

Broadcast on Friday 2 December 2011 at 1100 GMT on BBC Radio 4, or listen again on iPlayer

Capes guessed they had hit a mine. Finding that he could stand, he groped for a torch. In the increasingly foul air and rising water of the engine room he found "the mangled bodies of a dozen dead".

But that was as far as he could get. The engine room door was forced shut by the pressure of water on the other side. "It was creaking under the great pressure. Jets and trickles from the rubber joint were seeping through," said Capes.

He dragged any stokers who showed signs of life towards the escape hatch and fitted them and himself with Davis Submarine Escape Apparatus, a rubber lung with an oxygen bottle, mouthpiece and goggles.

British WWII submarine escapes HMS Umpire sank near Norfolk, England on 19 July 1941. Escapees: 14-15

HMS Stratagem sank near Malacca, Malaysia on 22 November 1944. Escapees: 10

HMS Perseus sank near Kefalonia, Greece on 6 December 1941. Escapees: 1

HMS P32 sank near Tripoli, Libya on 18 August 1941 (but the wreck was discovered only in 1999). Escapees: 2

This equipment had only been tested to a depth of 100ft (30m). The depth gauge showed just over 270ft, and as far as Capes knew, no-one had ever made an escape from such a depth.

In fact the gauge was broken, over-estimating the depth by 100ft, but time was running out. It was difficult to breathe now.

He flooded the compartment, lowered the canvas trunk beneath the escape hatch and with great difficulty released the damaged bolts on the hatch.

He pushed his injured companions into the trunk, up through the hatch and away into the cold sea above. Then he took a last swig of rum from his blitz bottle, ducked under and passed through the hatch himself.

"I let go, and the buoyant oxygen lifted me quickly upward. Suddenly I was alone in the middle of the great ocean.

"The pain became frantic, my lungs and whole body as fit to burst apart. Agony made me dizzy. How long can I last?

"Then, with the suddenness of certainty, I burst to the surface and wallowed in a slight swell with whitecaps here and there."

But having made the deepest escape yet recorded, his ordeal was not over.

His fellow injured stokers had not made it to the surface with him so he found himself alone in the middle of a cold December sea.

In the darkness he spotted a band of white cliffs and realised he had no choice but to strike out for those.

Story doubted

The next morning, Capes was found unconscious by two fishermen on the shore of Kefalonia.

For the following 18 months he was passed from house to house, to evade the Italian occupiers. He lost 70lb (32kg) in weight and dyed his hair black in an effort to blend in.

He recalled later: "Always, at the moment of despair, some utterly poor but friendly and patriotic islander would risk the lives of all his family for my sake.

Image caption Kostas Thoctarides and his dive team found the wreck of HMS Perseus in 1997

"They even gave me one of their prize possessions, a donkey called Mareeka. There was one condition attached to her - I had to take a solemn vow not to eat her."

He was finally taken off the island on a fishing boat in May 1943, in a clandestine operation organised by the Royal Navy.

A dangerous, roundabout journey of 640km took him to Turkey and from there back to the submarine service in Alexandria.

Despite being awarded a medal for his escape, Capes's story was so extraordinary that many people, both within and outside the Navy, doubted it.

Was he really on the boat at all? After all, he was not on the crew list. And submarine commanders had been ordered to bolt escape hatches shut from the outside to prevent them lifting during depth charge attacks.

There were no witnesses, he had a reputation as a great storyteller, and his own written accounts after the war varied in their details.

And the depth gauge reading 270ft made his story all the harder to believe.

John Capes died in 1985 but it was not until 1997 that his story was finally verified.

In a series of dives to the wreck of Perseus, Kostas Thoctarides discovered Capes's empty torpedo tube bunk, the hatch and compartment exactly as he had described it, and finally, his blitz bottle from which he had taken that last fortifying swig of rum.

Tim Clayton is the author of Sea Wolves: the Extraordinary Story of Britain's WW2 Submarines.

BBC Radio 4's Escape from the Deep is broadcast on Friday 2 December 2011 at 1100 GMT. Or listen again on BBC iPlayer.