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Natural Bridge Caverns, Texas
Credit: Natural Bridge Caverns.
Back in 1960, four college spelunkers journeyed below a 60-foot (18-meter) natural limestone bridge between San Antonio and New Braunfels and discovered Natural Bridge Caverns. Today, adventurous visitors can experience what it was like for those original explorers. On the park's Adventure Tours, guides will lead cavers as they climb, hike, rappel and explore on an excursion that's free of concrete and handrails only the light of your helmet will illuminate the way.
This is not for the weak-hearted, says the cavern's website. Cavers are outfitted with caving gear and lowered by rope through a 160-foot- (49-m-) deep well shaft and then continue on for an approximately 1-mile journey to 230 feet (70 m) below surface level to the Fault Room, which features one of the longest soda straw formations in North America at 14 feet (4 m) in length.