To live with ghosts requires solitude.
—Anne Michaels, Fugitive Pieces

FOR DAYS, I’d been searching Mexico’s Sierra Madre for the phantom known as Caballo Blanco—the White Horse. I’d finally arrived at the end of the trail, in the last place I expected to find him—not deep in the wilderness he was said to haunt, but in the dim lobby of an old hotel on the edge of a dusty desert town. “Sí, El Caballo está,” the desk clerk said, nodding. Yes, the Horse is here.

“For real?” After hearing that I’d just missed him so many times, in so many bizarre locations, I’d begun to suspect that Caballo Blanco was nothing more than a fairy tale, a local Loch Ness mons - truo dreamed up to spook the kids and fool gullible gringos.

“He’s always back by five,” the clerk added. “It’s like a ritual.” I didn’t know whether to hug her in relief or high- five her in triumph. I checked my watch. That meant I’d actually lay eyes on the ghost in less than . . . hang on.

“But it’s already after six.”

The clerk shrugged. “Maybe he’s gone away.”

I sagged into an ancient sofa. I was filthy, famished, and defeated. I was exhausted, and so were my leads.

Some said Caballo Blanco was a fugitive; others heard he was a boxer who’d run off to punish himself after beating a man to death in the ring. No one knew his name, or age, or where he was from. He was like some Old West gunslinger whose only traces were tall tales and a whiff of cigarillo smoke. Descriptions and sightings were all over the map; villagers who lived impossible distances apart swore they’d seen him traveling on foot on the same day, and described him on a scale that swung wildly from “funny and simpático” to “freaky and gigantic.”

But in all versions of the Caballo Blanco legend, certain basic details were always the same: He’d come to Mexico years ago and trekked deep into the wild, impenetrable Barrancas del Cobre—the Copper Canyons—to live among the Tarahumara, a near- mythical tribe of Stone Age superathletes. The Tarahumara (pronounced Spanish- style by swallowing the “h”: Tara- oo- mara) may be the healthiest and most serene people on earth, and the greatest runners of all time.

When it comes to ultradistances, nothing can beat a Tarahumara runner—not a racehorse, not a cheetah, not an Olympic marathoner.

Very few outsiders have ever seen the Tarahumara in action, but amazing stories of their superhuman toughness and tranquillity have drifted out of the canyons for centuries. One explorer swore he saw a Tarahumara catch a deer with his bare hands, chasing the bounding animal until it finally dropped dead from exhaustion, “its hoofs falling off.” Another adventurer spent ten hours climbing up and over a Copper Canyon mountain by mule; a Tarahumara runner made the same trip in ninety minutes.

“Try this,” a Tarahumara woman once told an exhausted explorer who’d collapsed at the base of a mountain. She handed him a gourd full of a murky liquid. He swallowed a few gulps, and was amazed to feel new energy pulsing in his veins. He got to his feet and scaled the peak like an overcaffeinated Sherpa. The Tarahumara, the explorer would later report, also guarded the recipe to a special energy food that leaves them trim, powerful, and unstoppable: a few mouthfuls packed enough nutritional punch to let them run all day without rest.

But whatever secrets the...