In the summer the glacial meltwater of the Alps runs through cracks in the rock and forms pools of water in the bottom of canyons. Tied to ropes fixed at the top, canyonists jump into the icy pools for what they claim is an astonishingly invigorating experience. It is normal for one person to climb down first to check the depth of the water, which can be up to a thousand feet. The deeper the canyon, the more the kudos.
Purists like to think it is "an underground thing". But the sport is already becoming commercial, with companies advertising on the Internet. The sport, however, has been banned in some states in America.
Phil Young, an "extreme sports" enthusiast, says "with drops of 100ft plus, it's pretty hard core". Like many "underground" sports, there is no official organisation to check equipment, offer advice or govern safety . Mr Young says that "there's probably some kind of league of canyons where you get more respect the more `epic' the canyon you've jumped".
Canyoning has become increasingly popular in Switzerland and other European countries, where the logistics of a trip are relatively simple. So far, however, it has not really caught the imagination in Britain.
The trip was organised by a company called Adventure World. Its Internet website advertises the experience for about pounds 40 and promises a day out to "explore an exciting world of unspoiled beauty by sliding through rapids, jumping off waterfalls and rappelling down cliffs. Our veteran guides will ensure your safety as you have the time of your life."
In Australia last April a British teenager, Siobhan Halls, 17, of Steppingley, Bedfordshire, was killed in the Blue Mountains, near Sydney. She died after hitting her head on rocks as she "rode" without proper equipment or instructors near by.
David Edwards, a physical education instructor, who has experience of canyoning said: "It is obviously very exciting. But it is also very dangerous. It is turning into a great craze, but there appears to be little thought given to things like safety. People taking part are diving into fast-running water past rock faces. One can imagine the potential for injuries and deaths."
As well as canyoning, Adventure World, based at Interlaken, also arranges bungee jumping events and rafting.