The Badwater 135, known as the world’s toughest foot race, began with 98 runners from 21 nations. Temperatures near 118 F and overnight lows around 90 F challenge the competitors. The race covers 135 miles and 14,600 feet of climbing.

The heat is severe, with temperatures expected to reach 124 to 127 F later in the week. Runners avoid the hot asphalt by running on the white line. Each runner has a support crew and vehicle for supplies, as there are no aid stations. Read more culture coverage.

The race cutoff has been reduced to 45 hours from 48, making the challenge greater. Nighttime temperatures in Death Valley have risen by about 10 percent over the past 20 years. Related: New Records Set at Badwater 135 in Death Valley.

This original Runner World brief is based on reporting from Marathon Handbook. Visit the source for its complete reporting, context and any later corrections.