BILLINGS, Mont. — Southwest Montana's soaring mountains and frigid rivers will define a rugged race course for more than 60 teams of endurance athletes beginning today, when a 500-mile cross country race kicks off in Big Sky.

Navigating with a map and compass, the four-person, coed teams must stick together as they run, bike, kayak, climb, swim and rappel a daunting wilderness course. With the competition intense, many teams are expected to forego sleep and push toward the finish around the clock.

Primal Quest Montana 2008 — touted by its promoters as the most challenging endurance competition in the world — offers a $25,000 prize to the first to finish.

For Primal Quest veterans, this fifth version of the event offers a sharp departure from the last race, set in the searing heat of Moab, Utah, in 2006. This year's event features a combined 100,000 vertical feet of ascents and lengthy stretches in rivers swollen by spring snowmelt — where water temperatures still linger in the 40s.

"The snow is melting rapidly and it's looking good on the higher elevations," said Kraig Becker, one of the race's organizers. "But of course the snow's got to go somewhere. We're keeping our eye on the whitewater sections."

If the water gets too rough, participants may face mandatory portages in some areas, Becker said.

In past races, 80 to 90 percent of teams have made it to the finish within the 10-day time limit.

Minor injuries are common — from blisters and dehydration to broken feet and toes. In 2004, an Australian contestant, Nigel Aylott, died after being hit by a falling rock in the Cascades mountain range in Washington.

Precise details of the course were disclosed Sunday. By keeping the route under wraps, organizers hoped to keep any team from gaining advantage through pre-race scouting.

Most teams come from the United States, with others traveling this year from Ireland, Canada, Mexico, the United Kingdom and Norway.

Only one team is from Montana. And a team sponsored by Nike is considered the favorite after winning all four previous races.

But while Montana might not be able to claim victory at the race's end, Donnie Sexton with Travel Montana said it offers good exposure for the state's tourism industry.

"It's pretty darned exciting — nothing I would want to do, but it's exciting," Sexton said. "I just have such a hard time imagining that people can do that kind of stuff 24 hours a day and barely sleep."