More than 300 people took part in the efforts to save the cavers. Photo: UWFRA

The worst tragedy in British caving history will be remembered this weekend.

It is 50 years since the events in Mossdale Caverns in the Yorkshire Dales in which six young potholers died.

The dangers presented by attempting to recover their bodies were deemed too severe and the six remain underground in the limestone system high on the fells of upper Wharfedale.

A memorial walk will take place on Saturday and a service will take place on Sunday at the tiny church of St Mary’s at Conistone, the nearest hamlet to the caverns.

More than 300 people were involved in the attempt to save the cavers, some from Leeds University, others members of local caving clubs, after the system flooded during heavy rain on 24 June 1967.

The memorial plaque at the site. Photo: Steve Partridge CC-BY-SA-2.0

Michael Ryan, 17; Bill Frakes, 19; John Ogden, 21; Colin Vickers, 23; Geoff Boireau, 24 and Dave Adamson, 26 all perished in the caves, 450m (1,476ft) up on the watershed between Wharfedale and Nidderdale.

Ten cavers entered the system about 2.30pm. Mossdale had a reputation as one of the toughest underground challenges in Britain, with nowhere to escape to if it flooded, and many of its passages at the top end of difficulty.

Four of the cavers, Collette Lord, James Cunningham, John Shepherd and Morag Forbes only went part way in, as far as Rough Chamber, before they retreated to the surface. The six others continued on their quest to use dynamite to blast a new passage.

Forbes waited for her friends to emerge at How Gill Nick, about a mile to the South-East. It began to rain heavily. She returned twice to Mossdale, and found Mossdale Beck in full spate and the cave entrance flooded.

The 22-year-old, who was Adamson’s fiancée, ran about 4.5km (3 miles) across the moorland to Yarnbury to raise the alarm.

Upper Wharfedale Fell Rescue Association members were called out shortly before midnight and were joined by colleagues from the Cave Rescue Organisation. Attempts to rescue the men lasted four days, and included bringing in an excavator to try to dam the cave entrance.

Volunteers dig frantically to divert the beck during the rescue operation. Photo: UWFRA

Fire service pumps were used to lower the water level, and the dam was maintained by the huge number of cavers and rescuers who had gathered at the remote site on Conistone Moor.

But the cave entrance was under four feet of water and it would be almost 12 hours before rescuers could get underground. The bodies of five of them were found about five hours later in the Far Marathon crawl, a long, 10-inch-high section of the system, but conditions deteriorated again and rescuers had to retreat to the surface.

Two days later, the body of the final missing caver, John Ogden, was found in a narrow fissure a short distance beyond where his five colleagues’ bodies lay.

Len Huff of UWFRA, one of the leaders of the rescue attempt, said: “Their luck just ran out. The weather changes quickly in this part of the world.”

Coroner Steven Brown returned a verdict of misadventure and ordered the cave be sealed as a grave, any attempt at bringing them to the surface ruled too risky.

The cave entrance lies on remote moorland between Wharfedale and Nidderdale. Photo: Bob Smith/grough

The men’s bodies remained where they had been found for another three years until a group of cavers broke through the seal and moved them to another chamber, which they named The Sanctuary.

A plaque now marks the place where British caving’s darkest day occurred. The memorial service will take place at St Mary’s at 6.30pm on 25 June.