Natural Bridge Caverns, Texas
1 of 10 CREDIT: Natural Bridge Caverns.
Carlsbad Caverns, New Mexico
2 of 10 CREDIT: NPS.
Lost Sea Wild Cave Tour, Craighead Caverns, Tennessee
3 of 10 CREDIT: The Lost Sea.
El Capitan Cave, Alaska
4 of 10 CREDIT: U.S. Forest Service.
Cosmic Cavern, Arkansas
5 of 10 CREDIT: Cosmic Cavern.
Fantastic Pit at Ellison's Cave, Georgia
6 of 10 CREDIT: Willie Hunt.
Great Blue Hole, Belize
7 of 10 CREDIT: NASA Earth Observatory.
Glenwood Caverns Wild Tour, Colorado
8 of 10 CREDIT: Glenwood Caverns Adventure Park.
Vancouver Island Caves, British Columbia, Canada
9 of 10 CREDIT: Horne Lake Regional and Provincial Parks/ www.hornelake.com.
Waitomo Glowworm Caves, New Zealand
10 of 10 CREDIT: Discover Waitomo.
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.