The fast-growing adventure sport combines skills and thrills of several other water sports and outdoor activities.

The sport is believed to have originated in France about 15 years ago, but its growing popularity has spread as far as Australia and New Zealand.

It is dangerous even in ideal weather conditions, and is banned for liability reasons in many American states.

Canyonists try to negotiate dangerous territory quickly

But the thrill of its inherent danger is what makes it attractive to those who take part.

"It's a real risk and there's danger to life and limb, but that is part of the attraction," says Nigel Shepherd, a qualified mountain guide and experienced canyonist.

It is the terrain that determines the means of progress, be it walking, wading, jumping, abseiling, climbing or swimming.

"All the time, with the water up to your knees or your waist, you must be careful in tackling slippery rock," is the advice of one canyoning tour organiser.

Experience required

The range of skills explains why some experts insist canyoning is only for those already experienced in basic outdoor activity skills.

"For people who have never done it before, the venue is chosen very carefully so it minimises the level of risk that people are likely to be exposed to," says Mr Shepherd.

Abseiling is a basic skill of canyoning

"Canyons will be chosen according to their degree of simplicity and their possible number of escape routes along the way."

Many organisers use qualified mountain guides as leaders. They insist those taking part are physically fit, able to swim and preferably do not suffer from vertigo.

Equipment includes life jackets, wetsuits, climbing gear and crash helmets.

Weather main danger

But absolute safety cannot be guaranteed. The main dangers of the sport are weather-related.

In April, three months before the disaster in Switzerland in which 18 people died, a British woman was killed canyoning in Australia's Blue Mountains.

Siobhan Halls, 17, died after hitting her head on a rock when she and her companions were swept away by a rain-swollen river. She had not been wearing a crash helmet.

Flash flood hazard

In 1996, a 43-year-old man from Ayrshire died when he was sucked into a whirlpool in the French Alps.

Even quiet, slow-running brooks can become dangerous white-water after a sudden downpour of rain. Flash floods are considered to be a major hazard in many canyons.

Easy abseils can become tricky and take much longer than anticipated in pouring rain.

Hypothermia is a major risk, particularly for inexperienced canyonists who are left waiting in cold water for an abseil to finish.

Tiring work

Deep ravines are often very cold places because sun-light is blocked out. Swimming and wading in cold water can tire out even the experienced in a matter of minutes rather than hours.

The favourite locations for canyoning are in the Swiss, Austrian and French Alps, the Spanish Pyrenees and in Australia's Blue Mountains.

Many companies have sprung up in recent years offering canyoning tours and introductory courses.

But there is also a growing number of thrill-seekers who set out on their own - often without the proper training and equipment.