Martin Schneekloth had never been to Chamonix, France, before, so his reaction to seeing a portion of the Western Alps for the first time on Tuesday was almost palpable.

“Driving in from the airport in Geneva, when you first see Mont Blanc, it’s quite exciting, but also scary at the same time,” says Schneekloth, a 44-year-old ultrarunner from Huntsville, Ala.

Schneekloth, who is originally from Germany, traveled 4,702 miles to this charming mountain hamlet with awe-inspiring views to run the Ultra-Trail du Mont-Blanc (UTMB), a painstaking 104-mile trail running race that circumnavigates the biggest mountain range in Western Europe. It’s a ridiculously hard course with more than 66,000 combined feet of elevation gain and loss and considered by many to be the hardest ultra-distance event in the world.

But Schneekloth, like so many other hardy runners from around the world, was attracted precisely by the sheer magnitude of that challenge.

If Ironman triathlons became the “what’s next?” after the marathon in modern society, this call of the wild—namely running freakishly long races through ungodly mountain ranges—appears to be what comes after that for those who like to challenge their physical, mental and emotional limits.

RELATED: UTMB Course Stats via Trail Run Project

Mountain ultraunning isn’t a new sport; it at least dates back to the first unofficial Western States 100 in 1974 (when Gordy Ansleigh famously ran 100 miles of an equestrian race between Squaw Valley and Auburn, Calif., when his horse came up lame and couldn’t run) and has been growing ever since. However, the notion of traveling around the world to run in some of the most notable trail races is a relatively new phenomenon—both for elite runners and trail-hardened age-groupers too.

Representatives from dozens of ultra-distance events from every corner of the world have been on hand at the UTMB race expo, trying to attract this week’s hardy participants to their race. Plus, those interested in running the UTMB and the other races in Chamonix need qualifying points earned in those races to gain entry.

“I’m here because of the challenge of the big mountains and the Alps themselves,” says John Hallsten, a 58-year-old carpenter from Helena, Mont. He’s been running ultras since the late 1990s and has five finishes of Colorado’s daunting Hardrock 100—largely considered North America’s toughest ultra— to his credit. To date, he’s run 17 100-milers and finished every single one of them. “I have wanted to come to the Alps since I was about 8 years old and this race gave me the opportunity to do that. I decided before I turned 60 that I’d come over and do this race.”

The 13th annual UTMB begins on Friday evening (noon ET in the U.S.) in Chamonix and will send more than 2,500 runners on an arduous journey around the Mt. Blanc massif and its namesake, 15,782-foot peak that towers over the region like a snow-covered skyscraper. It’s the fifth and final race during the week that annually celebrates the pinnacle of trail running amid the rugged mountains and breathtaking scenery in parts of France, Italy and Switzerland. Other race distances include the 50K Osieres-Champex-Chamonix (OCC), 101K Courmayeur-Champex-Chamonix (CCC), 119K Sur les Traces des Ducs de Savoie (TDS) and the 300K La Petite Trotte a Leon (PTL) and each one has a menacing amount of vertical relief.

It’s a reason most runners carry lightweight trekking poles and wear small backpacks with hydration, fuel and various pieces of mandatory gear.

“Yeah, it’s the sheer amount of vertical you can gain in any given mile that sets this place apart,” says American Sage Canaday, who is wearing race bib No. 1 in the UTMB because he’s atop the current International Trail Running Association (ITRA) rankings. “The trails are relatively smooth, but you could easily gain 1,000 feet in a mile and not even think about it. Every climb on the course is just very big and epic. It’s like 4,000 feet up, then 4,000 feet down on super steep trails.”

Even though he’s one of the world’s top mountain runners and owns a 2:15 marathon PR, Canaday is in awe of the local topography as much as anyone else. But he’s not the least bit sheepish about the challenge in front of him, even though this will be his debut at the 100-mile distance. He’s been training in Chamonix for almost a month to get prepared for the race.

“I say I like to try to run ‘any surface, any distance,’ so I wanted my first 100 to be a high-profile race on a big stage in an epic mountain environment where I thought the odds might be kind of stacked against me,” says Canaday, a professional runner sponsored by Hoka One One and numerous other brands. “But part of it was I really wanted to explore these mountains, too, because they’re so beautiful.”

RELATED: Virtual Trail Run of the UTMB Course

This is the biggest year yet for the UTMB festival of races. This past winter, 15,500 runners from around the world applied to enter one of the races, but only 7,250 were accepted. The lucky ones who made it hail from 87 different countries. Aside from France, which claims more than 5,000 runners entered in one of the races, Japan is next with 300 runners while China, Brazil and the U.S. all have more than 100. Officially, there are 171 American runners entered in one of the five races (including a record 90 in the UTMB).

“There’s a lot more climbing here in a short distance than anywhere in the U.S., so it beats up your legs a lot more on the way down,” says David Laney, a 26-year-old Nike-sponsored athlete from Ashland, Ore., who won the grueling Chuckanut 50K in Bellingham, Wash., in March and placed eighth in the Western States 100 in June. “I kind of like that because it forces you to power hike quite a bit, which saves the legs to some extent.”

Aside from the unrelenting terrain, the other big difference in Chamonix (and most of Europe) is the attention trail running gets from the local communities.

“It’s pretty incredible, because you feel the energy when you’re walking through town,” says Tim Tollefson, 30, a Nike-sponsored trail runner from Mammoth Lakes, Calif., who is running in the 101K CCC race. “It’s more than a just one-day event. It’s a weeklong festival of sorts and because of that you get more than just a spectacular race, there’s an entire cultural experience centered around trail running.”

Americans elite runners have had a good amount of success at the UTMB since its inception in 2002, when Topher Gaylord and Brandon Sybrowsky tied for second and Krissy Moehl won the women’s race and placed 24th overall among 67 finishers. Nikki Kimball won the women’s race in 2007, while Moehl won again in 2009. American Rory Bosio won the women’s race the past two years, lowering the course record to 23:23 in the process.

But plenty of top-tier U.S. trail runners have been destroyed by the mountains and forced to drop out or struggle to the finish line, too. That includes Hal Koerner’s epic 38-hour sufferfest in 2011 and Anton Krupicka’s struggles the last two years when he had been contending for the lead but was forced to drop out in 2013 and struggled to the finish in 48th place last year.

“I’m not really nervous for this race as I am with other races, because I don’t really feel like I’m racing anyone as much as I’m just going to be trying to get around the dang course,” says Stephanie Howe, a runner sponsored by The North Face who won the Western States 100 in 2014 and finished second this year. “I have some personal goals, but really I want to be able to smile and enjoy the journey. However that happens, hopefully it won’t take me too long to get back to Chamonix.”

Other top Americans include Darcy Piceu (UTMB), Sally McRae (UTMB), Jeff Browning (UTMB), Magda Lewy-Boulet (CCC) and Zach Miller (CCC). A few are here to race, most just want to endure the adventure and reach the finish line.

RELATED: Chamonix, The Culture of Hard-Core Trail Running

Every level of competitor, no matter how fast or how experienced, winds up in awe of the mountains after running in Chamonix.

“’Intimidating’ was my first thought, because I’m running through those mountains, but aside from that it’s really it’s just amazing here,” Schneekloth says. “The mountains, the terrain, the steepness of the trails. Everything is bigger here. Normally they say that about America, but when it comes to ultrarunning I think it’s true here in the Alps.”