Boundless

The "Burning Man" of the mountain biking world, this race attracts some of America's most eccentric mix, as competitors pedal a 24-hour loop course through the wild and dangerous cacti forest of the Sonoran Desert.

Set north of the Arctic Circle, this three-day cross-country ski race is considered one of the most challenging of its kind due to unpredictable weather conditions.

The athletes head to the Atacama Desert, the driest place on Earth for a 100-mile foot race that could be the mental and physical breaking point of the race season.

The Adventure Xstream Moab race in Utah is a riveting one-day, 60+ mile adventure competition. The race marks the first time Simon, Turbo, Rory and Hunter compete as a four-person team.

Deemed the hardest mountain bike race in the world, the MB Race Culture Velo set in the French Alps features 70km, 100km and grueling 140km course option with over 7,000 meters of single-track climbs, and harrowing descents.

This very demanding Quadrathlon race is amidst the scenic rolling hills of Aberfeldy, Scotland. The competition features a nearly one-mile swim across Loch Tay, 60 miles of fell running over seven hills above 3,000 feet, a seven mile kayak, and a 34-mile road bike.

The Boundless athletes take on the punishing Ultimate Mountain Challenge, the toughest event in the multi-day Vail Mountain Games festival.

The Superbowl of Obstacle Course Racing (ORC,), the Spartan Beast World Championship, brings the world's top OCR athletes together for a 38-obstacle course track in Squaw Valley, California.

The birthplace and world championship of the newest trend in ultra-racing, Ötillö is a unique race combining 10km of swimming and 65km of running across 26 islands in the Baltic Sea archipelago in Sweden.

The Bruce Peninsula Multisport Race is a 100km competition that features 16km kayak, 62km of treacherous road and trail biking and 21km of running along the beautiful Niagara Escarpment in Ontario, Canada.