Runners, skiers and bikers will set off from International Falls early Monday for the 16th annual Arrowhead 135 race.

More than 150 athletes are signed up for this year's race. It's a round-the-clock, 135-mile trek across the northern Minnesota wilderness — with minimal or no outside support.

Many years — including last year — Arrowhead competitors have faced temperatures well below zero. This year, the forecast calls for highs in the 20s above zero, which means softer snow.

"Soft snow is like walking on sand so it's more work to go slower — more friction on your feet, more friction pulling your sled," said race director Ken Krueger. "Certainly it's harder for the bikers — (for) bikers, the ideal is a hard hard flat trail; we had that last year."

From 2014 Photos of the brutal Arrowhead 135

The soft, wet snow also can be problematic for runners — but it might benefit those who've chosen to ski the 135-mile trail.

Whether it's bitter cold or soft snow, Krueger says the extreme challenge is part of the draw of the race.

"Most racers, given their choice, want to race in a really tough year," he said. "This is billed as one of the 50 hardest races on earth. And if you get here on an 'easy' year, it's almost like you feel cheated."

Last year about 35 percent of athletes finished the race; the average finish rate is less than 60 percent.

Biking is the most popular option, with running second and cross-country skiing third. Some competitors also choose an option allowing no outside support, even at checkpoints; they must carry all of their supplies from the start. (The standard policy is support only at checkpoints.)

The race ends at Fortune Bay Casino near Tower, Minn. The first finishers — on bike — are expected on Monday night. Runners and skiers will take much longer to complete the course. The cutoff to reach the finish line is 7 p.m. Wednesday.