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Carlsbad Caverns, NM - These desert caverns include more than a hundred separate caves, some of which can only be accessed as part of a tour using ladders and handheld lanterns.

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Carlsbad Caverns, NM - Bats exit the cave at dusk from May through October—a sight to behold!

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Blanchard Springs Caverns, AR - Here you'll find the world's largest flowstone and an underground river.

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Blanchard Springs Caverns, AR - The cave is considered a "living cave" because it's always evolving as nature takes its course.

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Jewel Cave, SD -

This national monument features lots of calcite crystals, which are its namesake jewels.

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Jewel Cave, SD -

The cave has more than 180 mi of passages to explore, making it the third-longest cave in the world.

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Craighead Caverns, TN - Also known as the Lost Sea Caverns, Craighead Caverns is home to the Lost Sea, the largest non-subglacial underground lake in the country.

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Craighead Caverns, TN - In human history, the caverns have been used as a Cherokee gathering place, a Confederate saltpeter mine, a location for illicit moonshine operations, and a dance club called the Cavern Tavern.

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Mammoth Cave, KY - Mammoth Cave is actually the world's largest known cave system, measuring more than 400 mi long.

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Mammoth Cave, KY - There are several options for tours and activities that will lead you through the winding passageways and chambers of the caves.

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La Jolla Sea Caves, CA - These sea caves on the California coast aren't really a spelunking opportunity, but visiting them via kayak is an activity not to be missed.

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La Jolla Sea Caves, CA - Picturesque and photo-ready, these caves showcase the incredible power of nature.

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Wind Cave, SD - A sacred space for the Lakota people, legend has it that this is where humans first emerged from the underworld and came to the surface of the Earth.

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Wind Cave, SD - The cave is known for its boxwork formations, a honeycomb-like pattern of calcite, as well as the way air moving in and out of the cave makes it appear to breathe.

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Caverns of Sonora, TX - Considered to be one of the most active caves in the world, more than 95% of the rock formations in the caverns are still in the process of growing.

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Caverns of Sonora, TX - The cave is known for its beautiful calcite crystal formations.

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Luray Caverns, VA - These beautiful caverns are full of rock formations and areas of water with dramatic names like Saracen’s Tent, Giant’s Hall, Dream Lake, and Tatiana’s Veil.

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Luray Caverns, VA - While you're there, check out The Great Stalacpipe Organ, which is a musical instrument that taps on stalactites to make different sounds.

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Ruby Falls Cave, TN - Deep inside Chattanooga's Lookout Mountain, you'll find a 145-foot underground waterfall. Welcome to Ruby Falls.

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Ruby Falls Cave, TN - In addition to the waterfall, the cave has lots of interesting rock formations to look at.

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Meramec Caverns, MO - They say notorious outlaw Jesse James and his band of miscreants once hid out here. 

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Meramec Caverns, MO - The limestone cave has colorful rock formations and an underground lake.

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Makauwahi Cave, HI - Hawaii's largest limestone cave is a site rich in fossils.

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Makauwahi Cave, HI -

Among the huge variety of wildlife surrounding and inside the cave, you'll find a whole underground ecosystem of blind cave invertebrates.

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Howe Caverns, NY - The second most-visited natural attraction in New York (it's hard to beat Niagara Falls!), the caves are full of strange rock formations, subterranean riverbeds, deep crevices, and an underground lake.

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Howe Caverns, NY - Tours start 156 ft underground.

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Natural Bridge Caverns, TX - These caves were formed thousands of years ago by an underground river.

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Natural Bridge Caverns, TX -

The site is named for a limestone slab bridge at the cavern's entrance, but lots of cool rock formations exist deeper inside the caves.

Sources: (Travel Zoo) (Atlas Obscura) (The Vintage News)

See also: Awesome canyoning destinations around the world

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