The queues build up from early morning, a mix of ramblers and curiosity-seekers. They join the tents of the local cavers who organise the event in a part of the Yorkshire Dales more noted for its heights than its depths.
I was more concerned with the straight drop of 365 ft directly below me. It felt like being on a scaffold, waiting for the wooden floor to be whipped away from beneath my feet. Instead of a noose round the neck I was strapped into a bosun's chair, hard-hat on head and careful to keep every extremity tucked in, as instructed.
'The first winch was set up in 1933,' Russell Myers, a member of the Craven Pothole Club, told me, 'and we've had no accidents so far.' The winch starts working at about 9am, or whenever the first visitors arrive. If you have to wait your turn you can wander the slopes of Ingleborough, have a hot-dog, or if the wait is a really long one visit the Ingleborough Show Cave, a 30-minute walk away. Russell Myers explains, 'I remember one August bank holiday when there was a four-hour wait by 9.30 in the morning, but it's not usually that bad. Some days during the week you might be lucky and turn up and go straight down.'
Eventually your turn will come, unless you've suddenly gone off the idea, as you might when you're sitting in the chair about to be lowered into the cavern. Then the floor's pulled away, the chair drops, your stomach stays where it was, and you descend into the darkening shaft. In front of your face the rock and moss flash by, and the smell of damp and earth gets stronger. Fell Beck splashes the 300 ft or more into Gaping Gill, creating Britain's longest straight-drop waterfall, before it makes its way underground to emerge alongside the entrance to the Ingleborough Cave and from there flows on through the nearest village, Clapham.
As you make your own way underground, the dark of the shaft becomes lighter again as you reach the main chamber, where paraffin lamps are set up to illuminate the cave. It's a breathtaking sight, and worth the wait. Some people say that the main chamber is big enough to hold York Minster, or even St Paul's itself. It looks big enough to me to hold both of them - 500 ft long and the roof disappearing 300 ft above your head.
Guides will tell you about Gaping Gill, and the network of caves that leads off it, even taking you along them for a glimpse of the torchlit world of the cavers. It's like being in a Jules Verne novel.
Most people spend an hour or so wandering round, hearing the stories of that first descent 99 years ago by a French lawyer, Edward Martel. He carried with him a lantern, a phone to his wife on the surface, and a flask of rum.
For those not convinced about potholing, Russell Myers offers a reassuring word: 'We do check that everyone's out again at night.'
Gaping Gill will be accessible from 20-29 Aug. Check last-minute changes at the Yorkshire Dales National Park Centre at Clapham (05242-51419). Cost: pounds 5. B & B in Clapham at the Brook House Guest House from pounds 13.50 pp (05242-51580)
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