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Billy Yang gets a lot of questions about why he runs 100-mile races. From the logistical (“Do you sleep?”) to the literal (“Are you crazy?”), people have countless questions for Yang and his fellow ultrarunners. But most importantly, they want to know why they do it in the first place—and always come back for more.

In his most recent documentary, The Why, Yang sets out on his third 100-mile race with camera in tow to get the answers. But he doesn't simply tell “why”—he shows us. In the 30-minute documentary, Yang takes on the Leadville Trail 100, one of the oldest and toughest ultras in the world. Speckled with shots of breathtaking sunsets and mountain vistas, Yang’s journey shows a portrait of both beauty and pain on the trails. With insights gleaned from other accomplished Leadville competitors, you slowly begin to unravel the motivations of the ultrarunning greats.

“It’s a real test of the human will and the human spirit,” professional ultrarunner Zach Miller said. “It takes you to places that a lot of other things won’t take you.”

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The miles are both emotionally and physically grueling for Yang. He contemplates humanity’s demise as comfort-seekers, our universal softening of wills, and our desire for the path of least resistance. He wonders if we’ve truly lost touch with our fearless, “tough as nails” hunter-gatherer ancestors. Can the ultramarathon reunite us with that primitive instinct to endure, suffer, and ultimately survive? 

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“When it’s your time, do you want to leave a well-preserved body, or do you really want to use it?" he asks. "A body with stories that says you’ve pushed it, and at times suffered, and you sought its potential.”

When Yang crosses the finish line 28 and a half hours later, he believes he has found more than 100 reasons to run 100 miles. But his number one reason might surprise you.