Running a marathon or ultramarathon is a testament to endurance and determination, but certain races are renowned for their grueling challenges. For the adventurous runner seeking to test their limits, here are four of the most daunting races in America.

Badwater 135, California

This 135-mile ultramarathon takes runners from the lowest point in North America, Death Valley, to the highest point in the contiguous United States, Mt. Whitney. The brutal course covers three mountain ranges, with a total of 14,600 feet of cumulative vertical ascent and 6,100 feet of cumulative descent. The extreme heat, arid conditions and relentless elevation changes make it seemingly impossible to complete within the 48-hour time limit.

Foot Levelers Blue Ridge Marathon, Virginia

Known as "America's Toughest Road Marathon," this race takes runners through the mountainous terrain of the Blue Ridge Mountains. Participants encounter challenging climbs, technical descents and rugged trails. The scenic beauty is a highlight, but the unforgiving hills and varying terrain make it a ruthless test of endurance. If 26.2 miles isn’t enough of a challenge, runners can participate in the Double Marathon.

Barkley Marathons, Tennessee

Considered one of the most brutal endurance races in the world, the Barkley Marathons is not for the faint of heart. The race consists of five 20-some mile loops that add up to an astonishing 60,000 feet of elevation gain, all to be completed within an unsparing 60-hour time limit. To add to the challenge, there is no marked route — participants must navigate the rugged wilderness using only a map and compass, all while searching for hidden books that serve as waypoints for each lap. If that weren’t enough, runners can’t simply follow someone ahead of them, as they’re sent in opposite directions at various points. It should come as no surprise that there are very few finishers each year.