Rescue workers in Switzerland up against raging waters

"The biggest attraction is that you kind of get yourself into this ravine, you lower yourself down on the rope, and then you are fully committed pretty much until the end," he says.

For the seriously committed, the end could be two days away with no "escape routes" - exit paths - along the way.

Nigel Shepherd: "There's danger to life and limb but that's part of the attraction"

"It's actually coming out of the other end and knowing that you have succeeded, that you have achieved, and not necessarily cheated death, but you have had a very, very exciting experience in a potentially life-threatening situation."

Nigel, 44, of Llanberis in North Wales, has been climbing into ravines, abseiling down waterfalls and riding water chutes for 25 years, before the term "canyoning" was even thought of.

Grew in 1970s

"It was just known as wild adventure. Gradually it became a popular past-time in the 1970s with outdoor pursuit centres that used to do things called gorge walks."

"You need to be able to react to situations very quickly"

But even with his wealth of experience, which includes certification as a qualified mountain guide, there is no room for complacency when entering uncharted territory.

"If it's a big, serious canyon, then there's almost a sense of trepidation about what you are about to undertake."

His first rule is to take stock of the surroundings, assessing hazards and how best to deal with them. He then settles into the compelling mind work of "rope work, abseiling and route finding".

Change comes fast

Danger is by no means always imminent, but conditions can change in a matter of moments.

"If there's a big waterfall you do not really want to get swept over that"

"You shouldn't really have to think quickly in the sense that you have to make split second judgements, but you do need to be able to react to situations very quickly if things start to go wrong."

The best practice is to continuously think how troublesome situations can be averted.

Flash floods

Fortunately, Nigel has never experienced the sort of flash flood that is thought to have killed 18 tourists on a canyoning venture in Switzerland. The prospect is a chilling one, he says.

"Clearly a rise of a metre in a narrow space of say 10, 20 feet wide is very significant.

"It's almost impossible to say that there is a standard way of reacting in a situation like that.

Churned around and around

"The best way is to try to get away from the level of the water as quickly as possible. But if you have got smooth rock walls on either side it's almost impossible to do that."

That leaves the option of riding the flow of the river and hoping for the best.

"Clearly if there's a big waterfall coming up then you do not really want to get swept over that, because the danger is that you get churned around and around within the eddy at the bottom of the waterfall and you will stay under."