While most of us like to think we're standing on solid ground, the reality is quite the contrary: The Earth just beneath our feet is kind of like Swiss cheese. It's riddled with holes, networks of them, carved into layers of stone by dripping water or eaten away by acid over millions of years. And when those holes have an opening at the surface of the Earth and are big enough for a human to climb into, they're called caves.
The most common caves are limestone caves. They've been eroded by mildly acidic water flow, often from rain or melting snow. Water becomes acidic when it mixes with carbon dioxide in the atmosphere. Limestone erodes easily, and there are tremendous networks of winding caverns underground in areas with limestone landscapes (a geological feature called karst). More rarely, caves result from intense occurrences like volcanic activity or the release of sulfuric acid from underground sulfur deposits.
However they're formed, caves can be incredible sights. Most have been slowly decorated over unimaginable time frames by the water that carved them out.
As temperature changes cause minerals to come out of solution, stalactites grow downward from the roof and stalagmites grow upward from the floor. The two meet in the middle to form columns. Minerals collect on walls, creating massive, textured murals, and water features dot the underground landscape. Sometimes, entire rivers run underground.
In this article, we'll take a look at five of the most amazing caves out there. Each contains incredible features that make them noteworthy, and most of them are open to the public -- even to the nonspelunking public.
First on the list is one of the most popular caves in the United States, drawing millions of visitors every year. And for good reason.
Carlsbad Caverns (United States)
Carlsbad Caverns National Park
New Mexico, United States
A whole other world lies beneath the Guadalupe Mountains in Carlsbad, N.M. It took millions of years to produce the enormous underground rooms that draw visitors to Carlsbad Cavern National Park from around the world.
With three known levels, at 750 feet (229 meters), 900 feet (274 meters) and 1,350 feet (411 meters) below ground, respectively, there's plenty for those visitors to experience [source: NPS]. The caverns seem decorated with some of the most stunning cave features around, including the Giant Dome, a column measuring 62 feet (19 meters) tall and 16 feet (5 meters) in diameter, and the Frozen Waterfall, a stone creation that bears an uncanny resemblance to its namesake [source: NPS]. There are stalagmites and stalactites of every shape and size, as well as basins lined with onyx crystals.
Still, perhaps the most shocking aspect of the Carlsbad cave system is its size. A single chamber in Carlsbad Caverns, aptly named the Big Room, measures 4,000 feet (1,200 meters) long and up to 625 feet (190 meters) wide and 350 feet (107 meters) tall [source: NPS]. And that's just what's been discovered so far: The caverns are still being explored and uncovered.
Up next: a cave of unimaginable dimensions.
Cave of Crystals (Mexico)
In Naica, Mexico, volcanic activity created conditions that would one day lead to an amazing discovery: A cave housing what may be the largest crystals in the world.
Cueva de los Cristales, or the Cave of Crystals, is a natural marvel of cause and effect. A volcanic eruption built Naica Mountain, depositing tons of anhydrite, which is a high-temperature form of gypsum. When the magma beneath the mountain cooled, the anhydrite dissolved into molecules that seeped down with the water. Beneath the mountain, those molecules came out of solution and crystallized, creating gypsum crystals that have grown to lengths of up to 36 feet (11 meters) [source: Lovgren].
These crystals are most likely unmatched anywhere in the world, due to the unique conditions of the Naica caves in terms of both water flow and temperature range. Some have compared the sight to Superman's home -- mammoth, glittering crystals jutting out from every surface. But only a relative few have seen the Cave of Crystals up close. Discovered in 2000 by a couple of miners, Cueva de los Cristales is part of an active mine, and it gets so hot down there that the cave researchers and journalists who have gotten a peek have had to wear full protective gear.
Next on the list: miles and miles and miles of caves.
Mammoth Cave (U.S.)
Mammoth Cave National Park
Kentucky, United States
It took 10 million years for Kentucky's Green River to create Mammoth Cave, and it shows: Mammoth Cave lives up to its name. It's the longest known cave in the world, by far: Measuring approximately 360 miles (580 kilometers) long, it's four times longer than the second longest cave, Optimisticeskaya in the Ukraine [sources: NPS, GORP]. And that's just the part of Mammoth that's has already been explored. Experts believe the cave might extend 1,000 miles (1,600 kilometers) [source: NPS].
It's not just extreme in length, either. Mammoth Cave boasts some interesting life forms, like eyeless fish, shrimp and beetles as well as spiders with no pigmentation. There are tremendous columns, like the 192-foot (59-meter) Mammoth Dome, and rivers running through the lowest levels of the cave.
Visitors to Mammoth Cave National Park in Kentucky can check out 10 miles (16 kilometers) of the extensive cave system.
Up next: a cave system with more than one world-famous component.
Mulu Caves (Borneo)
Mulu National Park
On the island of Borneo, visitors can tour some of the biggest and longest cave passages in the world, all within a single cave system. The Mulu Caves beneath Mulu National Park were carved out of limestone over millions of years, and so far, 125 miles (200 kilometers) of underground cavern have been explored; it's possible there's three times that distance waiting to be discovered [source: FDSM].
The Mulu Caves boast several honors. According to the Sarawak Forestry Department, Mulu has the largest passage, in Deer Cave; the largest chamber, Sarawak Chamber; and the longest cave in Southeast Asia in the form of Clearwater Cave [source: FDSM]. The cave system offers tours for everyone from beginners to experienced spelunkers. Those looking for a real expert experience can attempt the 10 or so hours of total darkness required to navigate Sarawak Chamber.
And last on the list, for those afraid of the dark, a cave with some natural illumination.
Waitomo Glowworm Cave (New Zealand)
Waitomo, New Zealand
In New Zealand, music lovers can go underground to experience some of the most memorable performances. The Waitomo Glowworm Caves have hosted musical events due to the system's incredible acoustics.
Specifically, the music venue is The Cathedral, a 60-foot-tall (18-meter) room on the cave's lower level that has hosted the likes of the Vienna Boys' Choir and Dame Kiri Te Kanawa [source: Discover Waitomo]. Its unique acoustics are said to produce an incredibly pure sound.
But that's not actually what the caves are famous for. As the name implies, the Waitomo Glowworm Caves have their own indoor illumination: thousands of Arachnocampa luminosa glowworms. The mosquito-sized creatures are found only in New Zealand and cast a glow over the cave's interior.
Visitors can particularly enjoy the glow during a boat trip down Waitomo River, which runs through the lower level of the cavern.
For more information on caves and related topics, look over the links on the next page.
The Pacific's Ring of Fire is a 25,000 mile long 'ring' that's home to much of the world's volcanic and earthquake activity. Why? Plate tectonics.
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More Great Links
- Carlsbad Caverns National Park. National Park Service.http://www.nature.nps.gov/geology/parks/cave/
- Gunung Mulu National Park. Forest Department of Sarawak, Malaysia.http://www.forestry.sarawak.gov.my/forweb/np/np/mulu.htm
- Lovgren, Stefan. "Giant Crystal Cave's Mystery Solved." National Geographic News. April 6, 2007.http://news.nationalgeographic.com/news/2007/04/070406-giant-crystals.html
- Mammoth Cave National Park. GORP.http://gorp.away.com/gorp/resource/us_national_park/ky_mammo.htm
- Mammoth Cave National Park, Kentucky. U.S. World Heritage Sites. National Park Service.http://www.nps.gov/history/worldheritage/mammoth.htm
- Mulu Caves (Gua Mulu) in Sarawak, Malaysia. Malaysia Central.http://www.malaysiacentral.com/information-directory/attractions-places-to-visit/caves/mulu-caves-gua-mulu-in-sarawak-malaysia/
- O'Hanlon, Larry. "Excavating the World's Basements." Discovery.http://dsc.discovery.com/convergence/planet-earth/guide/caves.html
- Waitomo Glowworm Caves. Discover Waitomo.http://www.waitomo.com/glowworm-caves-information.aspx