Ah, a spelunker's paradise! Caving, or spelunking, is a popular and dangerous sport among adventure-enthusiasts and explorers seeking the thrills of undiscovered deep, dark, and sometimes cramped spaces. Caves are mysterious and house a lot of geological history. But not all caving excursions need to be so, um, claustrophobic.

Here are some caves and caverns in the southern U.S. accessible enough that anyone can visit, yet crazy enough that visitors will be glad to have a tour guide — and a camera — by their side. 

1. Mammoth Cave National Park, Kentucky 

The longest cave in the world, Mammoth Cave is not just any old cave (though it is really old.) Visitors can take a number of tours through the cave system, namely an intense "Wild Cave Tour" spelunking adventure through miles of Mammoth Cave tunnels (shown in the video below.) For those not so spelunking-inclined, head southwest towards Bowling Green, KY and take a boat ride in Lost River Cave, where you can sit, listen to cave folklore, duck under low ceilings and explore cave rooms all from the comfort of your guided raft. 

2. Ruby Falls, Tennessee 

Head to Lookout Mountain, or rather in Lookout Mountain, and a long ride down Lookout Caverns where you'll find yourself walking down a dimply-lit limestone sculpted path towards a 145 foot tall waterfall tumbling into a pool of water at your feet. The explorer who discovered this slender free-falling waterfall named it after his wife, "Ruby." The sight is like a scene out of a movie —  with dramatic background music and soft color-changing lights to go with it. 

3. Cathedral Caverns, Alabama 

When you first start down the path into Cathedral Caverns, just one look at the unusually tall and wide entrance and you know that there are a lot of big formations hiding down in that cave. Head a little further down, and as you feel the muggy Southern air get colder, you'll start to notice the incredible stalagmites reaching towards the ceiling in what seems like colonies of oddly shaped columns. Keep an eye out for the shark tooth in the ceiling — proof that this cave wasn't always full of air. You'll have to go all the way to the bottom room of the cave to discover why it's called "Cathedral Caverns."

4. Luray Caverns, Virginia

400 million years in the making, the Luray Caverns offers some of the more unique formations to be found in a cave. If the chandelier-looking Saracen's Tent doesn't make your jaw drop, then the dream lake certainly will. The lake, no deeper than two feet, perfectly reflects the stalactites dripping down from the ceiling, and the mirrored illusion makes you wonder if you've just stepped through the looking glass into an entirely different world. 

5. Caverns of Sonora, Texas

It's not everywhere you can say, "Dude, check out that cave bacon!" The Caverns of Sonora have calcite crystal formations, most of which are still growing, that adorn the cave with formations such as speleothems, soda straw stalactites, and yes — cave bacon. The most famous formation of the caverns is the crystal "butterfly," which, unfortunately lost most of a wing from vandalism in 2006. 

6. Linville Caverns, North Carolina 

While comparatively small, the Linville Caverns in Humpback Mountain is an actively forming cave and is home to the Eastern Pipistrelle bat in winter and spring. It's not too shabby-looking, either...

7. Florida Caverns State Park, Florida 

The limestone formations in this cave are pretty cool and boast some neat fossils, but the park offers much more than just the 40-minute cave tours. Visitors can hike the park's nature trails, explore the museum, and picnic amongst rare plants and animals in the park. The cave itself has eight chambers; the last one decorated with Christmas lights. 

Bonus: Underwater Caves 

Breathing open air not really your thing? (With the proper cave diving certification) grab a SCUBA tank or three, a flashlight, and maybe an expert buddy or two and dive deep into these underwater caves. 

1. Wakulla-Leon Sinks Underwater Cave System, Florida 

Welcome to the longest known underwater cave system in the country. Maxing at around 300 feet deep, this cave system has at least 28 miles of passages. That's a lot of decompression time! The video below documents the dives that connected the Wakulla and Leon Sinks cave systems together, making it the fourth longest underwater cave system in the world. 

2. Hole in the Wall, Florida 

One of seven caves reachable by boat in the middle of Merritt's Mill Pond, Hole in the Wall has crystal blue waters and big white dome ceilings, some with "chandelier" formations. Divers should complete at least an introduction to cave diving certification before experiencing this cave which maxes out at around 100 feet deep. Once in the cave, look out for the "dragon's head" formation and the "mother of all great waters" tunnel —  a 35-foot-tall-room measuring over 1,000 feet long! 

--photos by istock/Andrew211, jollyphoto

--Allison Montroy is an editorial intern for Sierra and a journalism student at California Polytechnic State University at San Luis Obispo.