Trails come in many different shapes and sizes. There are the ones you saunter along on a lazy Sunday, carved out through the woods, and there are the ones that cut through the most challenging and exotic terrain in the world. Many of these latter group are home to intense, competitive races, and these are five of the most extreme.

The Badwater Ultramarathon

In a place like Death Valley, the one road serves the same purpose as a trail: it gives you a guiding path through an otherwise unnavigable terrain. Traffic isn’t so much a concern as guidance. And because of this, the Badwater Ultramarathon is one of the purest trail races there is: stay on the trail or your toast, literally. The Badwater is 135 miles long and passes through three separate mountain ranges between Death Valley and Whitney, California; not only is this an inconceivably long race, but it’s also through one of the least hospitable environments on Earth. If you really want to test your mettle — and especially if biking and swimming aren’t really your thing, ruling out an Ironman — look no further than Badwater.

The Barkley Marathon

The Barkley Marathon takes place in Tennessee’s Frozen Head State Park, and before you think, “Oh, it’s just another marathon,” consider this stat: since the race began, only about 1% of competitors have finished in 60 hours, the limit imposed by the race’s organizers. What makes it so difficult is, first, it’s extreme length — the Barkley covers over 100 miles — and second, the incredible climbs and changes in elevation. Over the course of the race, you’ll climb and descend almost 53,000 feet — that’s just less than two times the height of Mt. Everest. It’s about as different from the Badwater as you can get, but no less, and possibly even more, challenging.

The 6633 Extreme Winter Ultramarathon

Speaking of races that happen in the least hospitable environments in the world, how about the Arctic Circle? The 6633 Extreme Winter Ultramarathon takes place in the planet’s coldest place, covering either 120 or 350 miles depending on which race you choose. You have to carry all of your own supplies with you, which means traveling with a sled or backpack in addition to all of the clothing and gear you need to keep yourself warm. The race is so intense and hard to manage that they limit the participants to only 20 people, because otherwise the logistics and organization would be impossible.

The Arrowhead 135

Another ultramarathon, the Arrowhead 135 is slightly different than the other races we’ve covered so far: you can travel by foot, bike, and ski. Biking along frozen winter trails in Minnesota provides a special kind of challenge unlike many others in sports, and it’s so intense that on the race’s website, participants are encouraged to support and aid each other, particularly because they’re forbidden from receiving assistance from anyone outside of the race. And to keep racers safe, they have to finish with at least 3,000 calories of food and a certain amount of food and water as proof that they were keeping themselves safe and sustained along the way. The race goes through the night as well, requiring headlamps and sleeping gear. If you’re looking for the ultimate in winter challenges within the United States, this is your answer.

The Marathon Des Sables

Sure, Death Valley’s bad — but the Sahara’s worse. The Marathon Des Sables takes its racers 151 miles through the Sahara Desert, following pre-established trails through the fluctuating but always brutally hot and forbidding environment. The race takes 6 or 7 days and requires you to carry all of your own food and water, meaning that that has to be factored in to how you’ll travel the grueling distance without collapsing. With a race like this, it’s only partly about whether your body can cover the distance — it’s so difficult that it’ll push even the most physically fit people to their absolute limits. It’s also about whether your mind can handle the strain of a week crossing the Sahara with only yourself to rely on.