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Some 100 feet below the Earth's surface, BMX and mountain bikers are ripping up trails and soaring through the air in the cozy confines of a huge limestone cavern in Louisville, Ky. The cavern's owners say they've created the largest indoor bike park in the world, with 320,000 square feet of riding.

"There's, like, jumps everywhere you look," 9-year-old Tyler Bohm tells member station WFPL's Jacob Ryan. "I've never seen anything like this in a cave."

The Mega Underground Bike Park opened Monday as nearly 100 riders tried out the subterranean park, where the owners say the temperature is always around 60 degrees.

WFPL's Ryan shared some photos with us; back in December, a drone camera filmed a trial session.

The complex includes more than 45 trails marked by different technical skills and riding styles — and by features such as jumps and a cargo container that been integrated into a bike overpass.

According to Louisville's Courier-Journal, the course was designed and built by Joe Prisel of Burlington, Wash., who has worked on more than 20 indoor and outdoor courses.

The cavern was bought from a limestone mining company back in 1989, co-owner Jim Lowry tells WFPL, with the idea of opening a "high-security business park."

Around a dozen businesses are still using space in part of the cavern's 700,000 square feet, Lowry says. But in recent years, the owners have been trying to draw tourism to what they call the Mega Cavern. Lowry says they're hoping cyclists will be drawn to the new bike park.

When rider Alex Workman, who spent part of Monday soaring 20 feet in the air on the park's jumps, was asked by WFPL to describe the scene, his answer was short: "A playground."

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