Midsummer 1967. Six young men are two miles into Mossdale Caverns' labyrinth. They have passed the Drown or Glory swims, penetrated Rough and Kneewrecker passages and finally behind them is the 900ft Marathon crawl, a low and narrow tunnel only passable by wriggling sideways in the stream.

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Now hours from the entrance, they are heading for the distant end of the cave where they hope to find new passages. Next comes the infamous Far Marathon. Still, spirits are high. They are making good time. Some of them at least have been here before. They laugh, they tease each other. The omens look good for making a discovery. Then one perhaps mutters – is it a joke? – "Eh, what's that rumbling? Not the stream going up?" It is every caver's nightmare – lying flat-out in water, not a joke that cavers like. But now everyone hears it. Unmistakably, sickeningly, the water is surging; the draught whistles like a gale. As far as anyone knows – no one has tried it – Mossdale Caverns floods completely. Still they do not panic. They are the country's best cavers. Surely it cannot happen to them? The roof is a few inches above their prostrate bodies. They instinctively crawl faster, yet each of them knows there is nowhere to go. The oldest is just 26; others are still in their teens. The world of sunshine and fresh air has suddenly become an eternity away. United now by terror, inch by rising inch every caver's nightmare for them is coming true. It will happen very quickly. The cave will be their tomb.

On 24 June 1967, in the world's worst caving tragedy, six tough young men perished by drowning in the tortuous extremities of Yorkshire's Mossdale Caverns. Classed as Super Severe, the cave was notorious as being Britain's most testing – its far reaches then as now seen by fewer people than have walked on the Moon.

As the disaster made world headlines, most people in Britain were shaking their heads: another bunch of loonies gambling their lives for no apparent gain. For while the Sixties above ground were visibly swinging, deep under Britain some of its most daring sons were pushing back the boundaries of the unknown. They were mostly working class – this was DIY adventure for lads without much dosh. And with the planet's surface charted, caving suddenly was "in". Just up the road beckoned untrodden worlds. Unlike mountaineers, cavers face their challenges where nobody sees them, only making news when something goes wrong. In Harold Wilson's Britain, with few soldiers at war, cavers stuck in black hells made media epics that ran sometimes for days. Many were trapped students cast in life-and-death cliff-hangers; most ended with pictures of grinning escapees. But Mossdale broke the mould: a true-life horror story that few had expected – cavers drawn to their destruction by caving's glittering prize.

Mossdale Scar is a crumbly limestone cliff high on desolate Conistone Moor. A river-like stream, Mossdale Beck, disappears at its foot into the cave. Bleak and rarely visited, some think it looks menacing even in the sun.

A geological freak, the caverns are really two caves in one. Entered down a crevice, the accessible passages wind for almost seven miles, often only inches high, through shallow beds of Yoredale limestone where big caves should not exist. Yet most of the stream flowing into the cave disappears near the entrance into impenetrable rocks. It reappears five miles away, 18 days later and 900 feet lower, at a pool called Black Keld by the river Wharfe. The unexplored cave between the two points would be one of the deepest and longest in Britain. It is for some an obsession to find the fabled missing link.

Mossdale's fearsome reputation daunted most cavers from its first exploration in 1941 by iron-man Bob Leakey, caving's living legend, now 93. Driven by his fear of fear, he tackled chronic aquaphobia by founding the British Subaqua Club. Caving was his remedy for chronic claustrophobia. Soon making big discoveries, Leakey's resistance to cold and fatigue seemed super-human. Where mortals retreated, Bob launched an attack.

During the Second World War he stumbled on Mossdale. In a reserved occupation as an aircraft designer, with most cavers at war, he recruited as aides some game but unsuspecting aircraft-factory girls. Few, he admits, came twice.

Leakey's solo exploration of Mossdale Caverns was a feat of unimaginable courage and endurance – some still say madness – comparable in caving terms to reaching the South Pole. For trip after trip, using primitive equipment, candles and bike lamps, wearing only a boiler suit – often sleeping mud-caked in tiny flood-prone passages – he wormed through the cave. Had anything gone wrong, no one could have helped him. When there seemed nothing left to find, Bob had no reason to return; as for conquering fear, nothing left to prove.

Post-war cavers treated Mossdale gingerly. Caving then was tougher. With clothing bought at jumble sales offering zero protection, it took a fitness and resilience like no other sport. Devoid of stalactite caverns, Mossdale never would be fun. Some even doubted Leakey's account but not many relished testing it. For more than 20 years hardly anyone went near. Even its gauntlet epithet – the most strenuous cave known – attracted few contenders. Endless water-filled crawls with names like the Marathon, Kneewrecker and the descriptively painful Oomagoolie Passage made it, for most cavers, like a boxer so brutal that no sane person would take the fight.

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Another caving patriarch, Jim Eyre, now 82, swore that Mossdale was the one cave – a death trap in waiting – he would never go near. Ironically, in 1967, he found himself leading the perilous main rescue attempt.

But by the early Sixties a new generation, warmed by Neoprene wetsuits, was looking for laurels. Leakey, in his fifties, acquired two protégés, Mike Boon and Pete Livesey, of similar mettle to himself. In 1963, 22 years after their mentor, they were the first to reach again the end of the known cave.

Mossdale Caverns was unmapped. Neither Livesey nor Boon were patient cave-surveyor types – a tedious job in any cave but in Mossdale Herculean. Yet with no accurate map it was anyone's guess where the labyrinth was heading. Then another bunch of aspirants knocked on Leakey's door.

University caving clubs were generally ridiculed by caving's élite as cave-rescue fodder. But Leeds was the exception. The grandly styled Leeds University Union Speleological Society sought a reputation equal to its name. Mapping Mossdale Caverns – conceivably wresting its ultimate secret – would win the club cred like no other cave.

Starting only with Leakey's memory-drawn sketches it was a mammoth undertaking, made only just endurable by the new-fangled wetsuits that revolutionised caving. The club's David Adamson, getting to know Mossdale better than Leakey, identified places he believed would not flood. As well as prestige, the reward was new discoveries, minor but alluring, in the eerie muddy caverns at the end of the cave. Getting there was punishing but the boulder-chocked passages rekindled dreams of imminent breakthroughs. The fatal countdown had begun.

In the closely knit caving world rumours spread fast. From clubs across the North, young-tiger cavers were eager to gatecrash any Mossdale expedition. Brave, robustly fit, they knew they were good and, as trips became more frequent, felt increasingly invulnerable. Perhaps Mossdale's bark was worse than its bite.

Thus that Saturday morning the 10 young people who walked from Conistone village were an ad-hoc assemblage from three caving clubs. Only two had been down Mossdale before: 26-year-old Dave Adamson, the oldest and leader, and his friend Geoff Boireau, aged 24 and recently married, both from Leeds University. John Ogden, Jim Cunningham and John Shepherd, all aged 21, came from the Bolton-based Happy Wanderers Club. The others were Bradford Pothole Club members: Colin Vickers, 23, Bill Frakes, 19 – both top cavers – and Michael Ryan, 17, a promising beginner. Besides the men were Collette Lord, 19, accompanying the Bolton three for an unusual day out; and Morag Forbes, 22, engaged to marry Adamson the following month.

Adamson and Boireau's aim was to blast the blocked Mini-Cow passage. The others would be bearers, glad to drag gear and add Mossdale to their scalp list led by the man who knew the cave most intimately.

The weather was good but the forecast was dodgy. Thunderstorms seemed likely. Cunningham and Shepherd privately concluded it was too dodgy for them. (Both were due the following week, along with Ogden and Frakes, to join the British attempt to break the world depth record in France's Berger Chasm.) Adamson assured them he knew where to stay safe. Loath to lose face, there seemed no escape – but then the two women gave an excuse. Neither were cavers but they fancied an easy underground trip. Cunningham and Shepherd volunteered to chaperone if Adamson could spare them.

Today they remain adamant that if needed by Adamson they would have swallowed their unease. It was one of those casual but pivotal moments when an impromptu word determines who grows old and who dies young. The six drowned cavers so nearly were eight.

The party divided at the cave entrance. Adamson's team sprinted off with the explosives, Ogden ribbing Cunningham for using girls as a pretence for chickening out. Following with the group of four, Morag watched fiancé David disappear into the gloom.

The four emerged at 5pm. The drops of rain seemed insignificant. Cunningham and Shepherd drove Collette back to Ingleton. Adamson's six were not due before midnight. Morag sheltered in a barn. The rain became torrential. Checking back twice, she was alarmed by how fast Mossdale Beck was rising.

One inch of rain fell in three hours. When she returned at 9pm, shockingly – incredibly – a lake of swirling water covered the cave entrance. She ran desperately two miles to Yarnbury Farm where farmer Riley phoned the police. By 11.10pm the big cave rescue was on.

For the Upper Wharfedale Fell Rescue Team cave rescues were routine but Mossdale was the cave – the preserve of super cavers – for which no blueprint existed. It was the one the two leaders, Len Huff and Des Birch, had prayed that they never would have to face. Beyond their resources, they alerted the bigger Cave Rescue Organisation. Within minutes phones were ringing in cavers' pubs around the Dales.

One call was to the Marton Arms, near Ingleton, where Cunningham and Shepherd, oblivious to the Mossdale storm, were enjoying a pint. Grabbing still-wet caving gear they sped to the cave they had exited only hours before. From Yorkshire, Lancashire, Derbyshire and Cheshire, cavers joined the trek though the mud and driving rain.

Cavers rescue each other – no one else can – but for Cunningham and Shepherd no previous rescue had been so acutely personal. The scene was horrific. Mossdale was a lake; the cave underwater. Throngs of would-be rescuers sat fretting in the rain. The situation was unprecedented. No one knew where to begin. But soon a convoy of fire engines, JCBs and tractors were grinding up the moor.

As the midsummer dawn broke, the scene resembled a war zone. Hundreds slaved, many using bare hands, to dig a diversion ditch – six feet wide, 10 feet deep and 100 yards long – and build a 10ft dam, 15ft thick and 70 yards long. It was shaky, unstable – later reinforced by 10,000 sandbags – but created in one morning it was virtually a miracle: a race against time that many dedicated diggers believed already was lost. Then came the wait. Would it hold? Would water levels fall? If so, what then?

Jim Eyre, then 41, was among the most admired cavers in Britain. Adamson had asked him to join the expedition just a week before. Eyre was ready for most things but had always balked at Mossdale. Despite its potential, he was convinced it was a killer. He worried how nonchalantly young cavers were taking it; called the Mossdale mania Russian roulette with nature. Now, after labouring all night, he found himself waiting to lead the main team underground.

The dam required constant repair. Shifts of cavers crashed out in a village of tents. Watching the activity Eyre was never prouder of the motley individualists – often caving competitors – always ready to put their lives on the line for each other.

By late Sunday morning, helped by 19 fire pumps, the water was falling. A nervous Eyre led his sleep-deprived team down the still-gurgling cave. Cunningham and Shepherd, against advice that they were traumatised, insisted on joining Eyre's risky attempt with other Happy Wanderer members, John Rushton and Frank Barnes. Nicknamed the gentle giant, Barnes was perhaps the most committed of all: John Ogden was his closest childhood friend; at 11 years old they had explored their first cave. Frank knew that Ogden's father stood outside the cave desperate for good news about his only son.

Eyre's mission included establishing a phone link. Flood debris was everywhere. Foam filmed the roof. They crawled hearts in mouths as far as Rough Chamber, a room-sized cavern where it is possible to stand. The onward low passages to the notorious Marathon were ominously water-filled, sinisterly burping as air began to circulate.

Though all swear that Mossdale remains etched on their souls, 40 years on every version varies. But of those hours in Rough Chamber the enduring common memory is Eyre's indomitable humour. Twice as old as most of them, most conscious of the danger, his quipping reassured them of what he himself did not believe – that they soon would hear their trapped friends coming. Several thought they did: distant echoing voices, a tell-tale splash of water, darkness hoaxing nerve-racked ears.

No one had been in Mossdale before. Thoughts inexorably slipped to where the shaky dam outside was keeping them alive. But as the water slowly sank, they were joined by Tony Waltham, a London-based Wanderer who had rushed to the cave, astonished to find that no one else there had ever been to its end. At last they began the hardest journey of their lives.

Though still dankly dripping, Far Marathon now was abnormally dry. With no running water the silence was uncanny. To the leader of countless underground rescues it seemed like the silence of the grave. The 900ft crawl – 10 inches high and two feet wide – was too tight for passing if they met survivors. Eyre elected to let Waltham lead a few men forward but, frantic for their comrades, others pushed past too.

Waltham found the first two bodies jamming the passage. It was a terrifying moment; a nightmare to crawl over them. Just beyond he found three more. Of the sixth, there was no sign. It was a terrifying place. When Eyre caught up many of the team were weeping; several were retching; only Waltham spoke: "Go back, Jim. They're dead."

Phoning for guidance, Eyre was quietly told to speed everyone out. Outside things looked bad. The dam was visibly trembling. Twice it collapsed. Cavers in the stream formed desperate human barriers. The battle raged all night. Five corpses were located but where was John Ogden? Could the expert cave diver still be alive – maybe lost or unconscious – in some unknown high cavern?

As if destined to be part of Mossdale's alpha and omega, 26 years after he discovered the cave, rescue leaders consulted Bob Leakey. Early on Monday morning, the 53-year-old doyen led another team – again including Frank Barnes – to where he believed Ogden might survive. Reaching the five, he went far beyond, but with the dam again precarious, Leakey too was forced to retreat. It was his last caving trip.

Monday night and Tuesday the cave was combed again. Mossdale was world news. Sightseers augmented the rescuers and camera crews. It was becoming a circus but it was essential to determine John Ogden's fate. Two caver doctors signed declarations that Ogden had been flushed where he would never be found. But 10pm on Tuesday Ogden's friend Brian Boardman, another seasoned rescue leader, led a weary six-man team on a final attempt. Ever-hopeful, Barnes was there again. Beyond the bodies Boardman spotted a gold ring. Squirming around, he saw a boot protruding from a fissure. The cleft was heartbreakingly narrow – looked impossible to climb – yet Ogden somehow had fought there to the last inch of air.

They emerged into a bleak and overcast dawn. Ogden's father – who had hated his son's caving – needed only to look at Barnes's face. The battle was lost. Now nobody knew what to do next.

Rain was falling steadily; the dam still unstable. Reaching the corpses took five hours crawling – their retrieval seemed impossible. The coroner, Steven Brown, conferred with the parents, police and rescue leaders. Obtaining Home Office sanction for an inquest without bodies, he ordered the cave to be permanently sealed and be respected as a grave.

For parents and rescuers it was a painful decision. Whatever the cost, cavers get their man out. Most cavers, moreover, object to blocked caves – especially caves not fully explored. Many protested but four long days after the 10 had squeezed into the cave 300 rescuers exhaustedly went home, six comrades left unburied under Conistone Moor. The rescue report concludes: "Never has so much been done by so many people for such a small reward."

The following Sunday a quasi-funeral service was held in Conistone church. Hundreds of cavers packed the village lanes. Brown summed up his inquest that "there was great admiration for the courage and endurance of those who undertook this activity, but maybe courage and endurance in some cases exceeds discretion." The jury's verdict – misadventure. Though forgotten by the world, the tragedy remains caving's Battle of the Somme. Saying, "I was at Mossdale," is still like a badge of honour. Jim Eyre regards the victims as some of caving's fallen heroes.

Morag Forbes' bitter taste of the limelight lasted her a lifetime. She will today not discuss it. John Ogden's parents renamed their house Mossdale. Frank Barnes walks to the cave every anniversary: "A bit of us all drowned with them down there. We've had a life but they haven't." On the 40th anniversary he was accompanied by Jim Cunningham and many of the others with whom he shared those long four days.

Later that day Conistone village held a remembrance service. The vicar, Charles Knowles, comparing the drowned cavers with Donald Campbell, Marie Curie and Captain Scott, said that though in the wrong place at the wrong time it is a God-given instinct to go where no one's been before. A card on a bouquet reads: "Geoff Boireau – 40 years Geoff – always in my mind and heart – Judith, your wife." They had been married just one year.

Although officially sealed, in 1970 friends of the victims got into the cave and moved their remains to a high-level cavern, later named The Sanctuary, which may be out of the reach of floods.

Behind its brooding crag, 67 years after its discovery, Mossdale remains Britain's biggest caving challenge. Rumours abound. Some believe the Leeds club may have broken through but keep activities secret due to the cave's sepulchral status. Some like Simon Beck, 29, post blogs of their encounters. The cave, he admits, has become a fixation. A Mossdale Caverns map adorns his bedroom wall. His hero is Leakey; like him he explores mostly alone. He says Mossdale feels the loneliest place in the world; the farthest you ever have been away from home.

Aware of the risks, he monitors the weather – reckons he can beat it. Born 13 years after the tragedy, he admires and relates to the '67 six but doubts he is as good. Out of respect he avoids their burial cavern but is sure they would want continued exploration. Mossdale feels malevolent but its greatest threat is fear. Leakey says that Mossdale, like an underground Everest, always will be there. Cavers will get in. Cunningham admits if he were 20 that he would be among them. Eyre jokes that perhaps the sole advantage of being 82 is that no one would expect him to go down again. Mossdale will always pit man against cave – despite modern equipment, sooner or later a cave like Mossdale always wins.

Ray Kershaw presents 'The Mossdale Story' on BBC Radio 4 this Wednesday at 11am