Those twilight nights of winter are so quiet and tranquil. Or are they?  We found something different at Purgatory Resort in Durango, Colorado.

A pair of headlamps  bobbing oddly through the woods, getting brighter. The sound of crunching snow grew louder. Then came the breathing. A few hundred yards away, fat bikers with night lights came careening down a snowy slope, dropping a staircase into an empty courtyard that bustles with skiers and tourists by day. Just then on the snow, a group of Nordic skiers glided by. Welcome to Twilight Nights.

The McDonald’s Twilight Nights Race Series is a combination of three different courses and three different disciplines.

For four Wednesdays in the middle of winter at Purgatory Resort, Colorado’s base area, Nordic skiers, fat bikers and snowshoers gather under a sinking sun to duke it out in the dark.

Here, we chase these racers and race organizers through frigid temperatures to see what brings them out of their warm winter dens.