GENEVA — One morning in late June, 1,200 runners snaked up the south side of the Chamonix Valley before dawn in southeastern France. They were participating in the Mont Blanc 80 KM, a grueling running race in which participants would cover 50 miles on foot.

The distance, though, might have been the easy part.

They would also climb a total of 20,000 feet in ascents over the terrain, dip into Switzerland and cross a glacier before dashing across the finish line — all in less than 24 hours, the race’s time cap. (Alex Nichols of the United States went on to win the race in 10 hours, 31 minutes.)

The racers are part of the growing sport of skyrunning, an extreme discipline in which runners speed up and down mountains. Over the past 10 years, the Mont Blanc race series, one of the sport’s most iconic events, has seen a 315 percent increase in participants, from 1,600 racers in 2005 to 6,635 in 2015.

Whereas trail running includes running on trails in any natural setting, skyrunning combines mountaineering and running, and takes place exclusively in the mountains. It is distinguished not so much by its distance (races range from 2 to over 50 miles), but instead by vertical gain and the altitude at which the discipline takes place, almost always at or above 6,500 feet. Skyrunners might run on trails, but they often forge their own routes across meadows and rock fields, or up and over rock faces.