A few years ago if you wanted to zip line you had to go somewhere exotic like Costa Rica. Today you can find zip lines in tourist spots all over the United States with more popping up all the time. Most are in scenic destinations where you’ll zip through wooded areas, green with foliage, high above the earth. But there is only one place in the world where you can zip line below the earth’s surface, and that’s in the Louisville Mega Cavern.

The dimly lit dank atmosphere adds an element of eeriness as you zip through six caverns with names like Cavern to Hell and make your way across three narrow, swaying suspension bridges high above the cavern floors. At times 70 feet of air separates you from the ground. The last zip of the tour is a dual zip, from which you launch on seemingly the highest, smallest platform of the course and then race a fellow zipper through the canyon.

The 100 acre Louisville Mega Cavern, considered Kentucky’s largest building, was a limestone mine from the early 1930s to the early 1970s, the crushed limestone used for road paving. Some of the mine’s 4,000,000 square feet has since been filled in with inorganic materials like Corvette car bumpers, roofing shingles and plastic keyboard keys. Those areas have been converted to businesses, popular for warehousing critical records since the cavern can withstand tornado winds of up to 260 miles per hour. But a section of the cavern, the portion with the zip lines, was left as it was after the mine ceased operations. This portion of the cavern looks like any other natural cave, sans stalactites and stalagmites, which are just beginning to form. You’d never know when you’re in the cavern that you are right below an expressway, a zoo and even a Kmart.

The cavern’s natural temperature is always 56 degrees, so you’ll be comfortable no matter what the weather is outside, even with the ten pounds of harness you’re required to wear. And since you’re underground, you have no worries about being rained out.

If you don’t care to zip, you can tour the cavern in a tram. A wheelchair tram is available for those who need assistance, but you should call ahead to reserve it. Louisville Mega Cavern is located at 1841 Taylor Avenue in Louisville, with plenty of free parking available.