Deep in the Earth, perhaps under your very feet right now is another world awaiting exploration—caves. Caves are natural openings usually found within rocks that are formed by a variety events like volcanic activity or water flow throughout thousands of years. This creates several different cave types including: volcanic, glacier, crevice, erosion, and solution. While some caves are too dangerous to visit, many caves have become beacons of tourism for the areas. Spelunkers of all skill levels flock to them to have a look at the passages that could lurk under our feet.

Carlsbad Caverns in Carlsbad, New Mexico, US

Carlsbad Caverns is the crown jewel of the Carlsbad Caverns National Park. It is famous for its Big Room which was once thought to be the largest naturally created underground chamber in the world. While the Big Room has since been ousted as biggest chamber in the world, the caverns itself remains a UNESCO World Heritage Site and a favorite hotspot among vacationers.

The Carlsbad Caverns lays under the Guadalupe Mountains and is often described as the "Grand Canyon with a roof'. What thrills the many visitors of this cavern is that it is a solution cave, a common type of cave in the American Southwest. Solution caves are limestone-based caves that were carved out by water over the course of millions of years. Due to the constantly dripping waters, finding stalagmites and stalactites is common place. Carlsbad Caverns is no different, it's fierce looking stalactites hang from the room like daggers and the stalagmites rise up like vicious teeth.

While these caverns runs over 30 miles, that scientists know of anyway, only the first three miles of the cave are available for touring. The most spectacular sight of Carlsbad Caverns can be seen at the mouth of the cave at sunset. When the sun goes down, over a quarter million of Brazilian free-tailed bats spiral from the mouth of the cavern like something out of a horror movie to begin their nightly hunt for insects. The major downside of this particular enclosure is easily its popularity, this place is usually packed with vacationers in the summer seeking the awesome sights and cool refuge of Carlsbad Caverns.

Harrison's Cave in Barbados

Harrison's Cave is counted as one of the 7 Wonders of Barbados and is the island's most famous attractions, aside from the crystal clear ocean and white sand beaches, that is. Locals have known of this magnificent cave since the 18th century, however an official exploration of it was not launched until the 1970's when speleologist Ole Sorensen immediately recognized it's potential as a soon to be beloved attraction. After the official exploration, work on the cave commenced to improve lighting and divert streams to create trails within the cave. Now Harrison's Cave attracts scientific, artistic, technological, and merely curious visitors in the thousands each year.

Like Carlsbad Caverns, Harrison's Cave is a solution type cave where water carved the underground sanctuary from the limestone. This has also made Harrison's Cave a gallery of stalactites and stalagmites. Guests are taken into the Harrison's via electronic trams where they can walk alongside the crystal clear streams that plunge into beautiful emerald pools via waterfalls.

While Harrison's is a popular tourist attraction on Barbados, it is a relatively lesser known cave. However, it is a great respite for island visitors that have taken in too much sun and surf, yet have no desire to return to their hotel.

Waitomo Glowworm Cave in Te Kuiti, New Zealand

While not the biggest, longest or most popular cave in the world, the Waitomo Glowworm in New Zealand is perhaps the most interesting enclosure in the world. The cavern's name comes from a combination of the Maori word for big hole and the creature that has made its home in this cave system—the glow worm. The glow worm, or Arachnocampa luminosa, is a creature unique to New Zealand. The walls of this cavern are covered with this tiny creatures, as well as flowers, fungi, and giant crickets. The glow worm radiates unmistakable bluish green light that makes the dark ceiling look more akin to the night sky. The biggest attraction of Waitomo Glowworm is easily the serene boat ride through the underground lake that takes guests under the light of thousands of glow worms that cling to the ceiling.

The Waitomo Glowworm was formed over thousands of years of geological and volcanic activity. Many of the limestone formations of this cave were formed when New Zealand was still under the ocean waters around 30 million years ago. Due to this soggy beginning, there are many examples of fossilized coral, fish and sea shells within the walls, making this not just a tourist hotspot but a archeological one as well. Sadly, eager archeologists are not allowed to dig around due to the fragile ecosystem within Waitomo. The Scientific Advisory Group of New Zealand frequently monitors carbon dioxide levels, rock and air temperature, and humidity to help keep this place and it's delicate flora and fauna intact for as long as possible.

Chongqing Cave in Chongqing Province, China

While Chongqing Cave, or Er Wang Dong as it is locally called, may not have much in the way of animal life like Waitomo it does still have quite the vibrant ecosystem. Within Chongqing Cave, there are rapidly flowing streams, lush shrubbery, and it even has its own weather. Initially, the locals used the entrance of Chongqing Cave as a nitrate mine, however they never fully explored the cave because they physically couldn't. In fact, no one explored this cave until 2012 so no one realized how massive it truly was. The only way into the rest of the labyrinth was a tiny hole with an around and 800 foot drop to the ground. The team of 15 speleologists that dared to explore this enclosure found it to be a massive behemoth. With merely the first chamber having an 800 foot high ceiling and a width the size of 12 foot ball fields, it has made this chamber, which is now named Cloud Ladder Hall, the largest cave chamber in the world. The Cloud Ladder Hall and some of the other chambers are so high that they have even developed clouds. The fog that settled in the chambers often gets trapped on the ceiling when it tries to rise, resulting in clouds and occasionally rain.

Unlike the other caves presented here, Chongqing is not yet fully explored or open for tourism. So while this massive gem in innately interesting, spelunkers will have to wait awhile before they can visit this one.

Mammoth Cave in Mammoth Cave, Kentucky, US

We have gone through the most interesting and the biggest cave chambers in the world, now it is time for the longest cave. Mammoth Cave is located inside Mammoth National Park in Kentucky. It is the world's largest network of natural caves and underground passages in the world. As such it has been named a UNESCO World Heritage Site. With over 285 miles of passageways explored, there are miles of the cave that have never been seen by human eyes still today. The cave is also home to 130 varied types of flora and fauna, many of the these species are endangered but have found refuge within this massive enclosure.

Mammoth demonstrates 100 million years of cave-forming action and has been a prized site for tourism for years. However, Mammoth has also been the center point of local legends for years before it became a hotspot for spelunking visitors. Originally, it was mined for its saltpeter reserved in the 1800s. However, the cave developed a reputation for being haunted after several Native American mummies were discovered as well as more than a few stories of people becoming lost in the massive labyrinth. However, today those stories only add to the interest to eager explorers. Will they find a new passage not on the map? Or perhaps the remains of another Native American or lost soul?