INTERLAKEN, Switzerland (AP) _ When a flash flood killed 20 adventurers in a Swiss gorge last week, the obscure sport of canyoning was branded a dangerous thrill. But among Alpine activities, it's far from being the No. 1 killer.

Switzerland's mountains have long drawn daring souls to ski its steep slopes, climb its lofty peaks, hike its rocky trails and paraglide off its high cliffs.

The country has some of the Alps' most challenging mountains that attract hordes of climbers, not all of them properly equipped. The Eiger and the Matterhorn alone claim the lives of a handful of people each year.

In all, 106 people died in the Swiss mountains last year, half of them foreigners. The dead included 33 skiers killed in collisions, avalanches and other accidents, as well as four mushroom-pickers and two shepherds.

But few of those fatal incidents approach the sheer scope of Tuesday's tragedy, which killed three guides and 17 adventurers in what is believed to be the worst canyoning accident ever. One participant remained missing Sunday and was presumed dead.

In canyoning, participants clad in life jackets, wetsuits and helmets jump off cliffs into tiny mountain pools and shoot through rapids and over waterfalls.

The deaths prompted Swiss authorities to wonder openly how far vacationers should be allowed to go to satisfy their desires for danger.

``Maybe we should learn again to respect the limits set by nature,'' a Swiss government official, Samuel Bhend, said after the accident.

But the authorities are unlikely to stop pleasure-seekers coming to Swiss resorts for more traditional pursuits, such as skiing and walking _ all of which account for far more accidents each year than canyoning.

Rega, which operates air rescue services in Switzerland's mountains, said it dealt with five canyoning accidents last year, compared to 842 for skiers, 309 for mountain hikers or walkers and 53 for climbers.

About 150,000 sporting accidents occur in Switzerland every year. The Suva insurance company said that nearly 27,000 of those accidents involve skiers and more than 9,000 involve snowboarders.

Injuries have risen sharply for snowboarders and in-line skaters as those sports have gained popularity, Suva said, but the combined numbers for adventure sports like canyoning and bungee jumping remain much lower.

Canyoning ``isn't any more dangerous than skiing,'' if participants are properly accompanied, said Pierre-Alain Hoffer, a spokesman for the Swiss Mountain Guides Association. ``In this environment, it's no more dangerous than any other sport.''

``Of course an accident can always happen,'' he said. ``Zero risk doesn't exist in these sports.''

A series of accidents involving climbers last summer raised concerns that many people still seriously underestimate the dangers of that sport.

The chief of Zermatt's rescue services, Bruno Jelk, said two Spanish tourists even set out in sneakers to climb the 14,690-foot Matterhorn.

The Swiss Alpine Club says 1,311 people called on mountain rescue services last year _ 1,210 of them hikers or climbers and the rest hangliders or paragliders.

Swiss tourist officials said most adventure tour organizers were suspending canyoning trips until the cause of the disaster was determined.

Although tighter regulations on canyoning will likely be considered, there are no immediate plans for a clampdown on that or other extreme sports.

``As far as the government is concerned, we can't take care of all possibilities in advance by limiting them or preventing them on security grounds,'' said Oswald Sigg, a spokesman for the sports ministry.