What motivates athletes to do something incredible?

The reasons for seeking to achieve the impossible are about as vast as the athletes who set out to do so. Ultramarathoners make up one distinct group of such thrill seekers that has been growing over the last few years—but exactly why can be hard to pinpoint.

Maybe it’s because society is beginning to focus on fitness. Or maybe we’re obsessed with pushing the envelope. Whatever the reason, something drew more than 10,000 runners to the French Alps for this year’s edition of The North Face Ultra-Trail du Mont-Blanc.

The Ultra-Trail du Mont-Blanc is one of the ultramarathon community’s most famous events. Athletes are challenged to complete the 166-kilometer course—one that snakes through the mountains, hitting elevations of around 9,600 meters. The feat requires more than 20 hours of running for the best athletes, and between 30 to 40 for many others.

At the end, there is no trophy or prize money, just the cheering of the crowd and a pat on the back. So why is such an event growing every year?

According to Rory Bosio, Timothy Olson, and Hal Koerner, the reason is simple—curiosity.

This 13-minute film documents the trials and tribulations of the three runners who teamed up for the 2014 Ultra-Trail du Mont-Blanc. They could not be more different. Rory is naive and quirky. Timothy is an ex-junkie with something to prove. Then there’s Hal, the pretty-boy ultramarathoner who hits the trails with a perfect smile and Hollywood-like persona.

The film shows the ups and downs, and seeks to explain what makes ultrarunning so appealing, addicting, challenging, and ultimately, rewarding.

Watch The North Face’s short film, Curiosity, below: