By Ben Kesling 

CRYSTAL CITY, Mo.-- Zach Waske went through his pre-race routine, tightening his running shoes, scratching his beard and doing calisthenics.

Also, he adjusted his headlamp.

"It's gonna be a tough race," said the 30-year-old aircraft-parts salesman for Boeing Co., as he surveyed the eerie gloom of a 200-acre sand mine about 150 feet below the snowy woods of this town about an hour outside St. Louis.

Conditions were perfect, as usual. Fifty-six degrees and pitch black in places, but the veteran cave runner was nervous. You never know who might sneak out of the dark to steal a victory.

"Sometimes you can pull it off, other times there's somebody from out of state who blitzkriegs it and wins in record time," he said.

Mr. Waske was among nearly 900 runners competing in the fifth annual Sandmine Challenge, a race that generally falls into the category of obstacle course races or mud runs, increasingly popular pastimes for people bored by flat-surface races.

Now cave running, or speRUNking, is taking the trend to new depths. Racers run though loose sand, duck under rock arches and wade through chilly, waist-deep water guided through much of it only by the light of their bobbing headlamps.

"There's a lake and that scares me," said Nicole Tyler, a racer from Springfield, Ill., who stood about a head shorter than most others on the course.

At 10 a.m., Mr. Waske took off in the first wave of runners, including a couple of friends who run every year. Among the group, Jack Bourbonnais, a lean runner and perennial top finisher hungry for the title. They zipped around a dark corner, hitting the first water obstacle. Splashes, whoops and curses bounced off the water and the rock walls.

Above-ground obstacle races have ratcheted up their intensity over the years. One of the most popular, the Spartan Race, presents a "riddled battlefield of insane terrain" where racers tote boulders and jump through fire. The Tough Mudder race zaps competitors with electric shocks, fills their lungs with noxious gas and plunges them into ice-cold water, but all of that is done in the light of day.

Here, the dark adds a special challenge.

Even experienced runners like Mr. Waske hit snags. He and his friends were setting a scorching pace when they ran into a dead end.

"We've got giant sandstone pillars on each side," Mr. Waske said, recalling the stony cul-de-sac. "As soon as we hit it, we said, 'What did we do? Maybe we missed something.' "

They searched for a way out with their headlamps before realizing that a marker had been moved. They soon got back on track by following music wafting over from the finish line.

"It's easy to get turned around back in there," said Jake Goldsborough, director of race productions at Fleet Feet Sports in St. Louis, who had spent several days setting up the nearly 3-mile course. He remedied the situation by grabbing a sledgehammer and banging a spike into a sandstone pillar to post caution tape to guide runners.

More than an hour later, as Mr. Waske and his friends sipped on post-race beers, more than two-dozen college students bounded off in some of the last waves of the day.

Among them: Andrew O'Sullivan, a freshman at Washington University in St. Louis and the vice president of the school's running club.

The shirtless 19-year-old neglected to bring a headlamp, instead toting a dinky LED flashlight his mother had put in his Christmas stocking. "Lucky it was waterproof," he said. "There were times when it was fully submerged."

The water just a few hundred yards from the start took its toll on him. "It was kind of demoralizing to hit that right at the start," he said. "I was expecting to hit some other obstacles first and kind of ease into the race."

Ms. Tyler's fear of the water also proved daunting. "At certain parts, it was up to my chest because I'm so short," she said. It took her breath away.

As Mr. O'Sullivan hurdled rock walls and ducked through old pipes, racers from prior waves gathered for beers near a DJ at the post-race party. There, in a blue Hawaiian shirt, sat Tom Kerr, owner of the cave known as the Crystal City Underground complex. He hosts barge rides on the cave's lake, various sporting events and parties that sometimes clash with aboveground authorities.

"I got no roof costs," he said smiling. "Buildings have pipes that can freeze and break in the winter; I've got none of that."

In early February, a municipal judge found Mr. Kerr guilty of holding improperly planned underground "raves" in the cave last year.

He says he will appeal the ruling, arguing that since it is a cave it should be regulated not by local authorities, but by Missouri mining officials.

Meanwhile, Mr. O'Sullivan headed for the finish.

"I had pure excitement and adrenaline running through my body and had my flashlight ready," Mr. O'Sullivan said later. He vaulted over a waist-high wall and scrambled up a ladder, leaping from the top to save time. "I didn't get turned around too much."

He finished in just over 16 minutes, beating Mr. Waske by a minute, topping Mr. Bourbonnais, and winning the race. He and his 26 teammates didn't even wait for the awards ceremony. They just hurried to their cars and headed off for lunch.

Mr. Waske, still in the cave sharing beers with his friends, took his seventh place finish in stride.

"As it catches on, you start to attract people like the running team from Wash U," he said. "We'll always be ready to give it a go next year. Maybe they can throw in some more obstacles."

Write to Ben Kesling at benjamin.kesling@wsj.com

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