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There are monsters in the basements of the world: weird white crabs, pallid blind salamanders, hydrogen-eating bacteria and worms that glow in the dark. They live in caves furnished with bizarre and beautiful crystal chandeliers, limestone turrets and eerily dead-calm lakes.
It's a world without day and night and where the only circadian rhythm may be the coming and going of bats or birds, which supply the guano — the manna on which most other cave life depends. These outsiders are a conduit for nutrients that ultimately come from a sun the cave-bound creatures never see.
No one knows how many caves there are on Earth. Probably millions. Many are undiscovered and others are only partially explored. Likewise, there's no telling how many species of specialized cave organisms are out there that have yet to be discovered.
Just in the tristate area of Tennessee, Alabama and Georgia, for instance, there are about 15,000 caves, not including hollows less than 50 feet long, says cave conservationist Lynn Roebuck of the National Speleological Society.
Swiss Cheese
The reason so many places on the planet are riddled with caves is that they all share an abundance of limestone. This rock is rather easily dissolved by even slightly acidic water, which is common enough from water interacting with air and soils.
The limestone itself comes from eons of the remains of shells and skeletons of sea creatures piling up on a seafloor. This means, of course, that all limestone was once at the bottom of long lost seas and was later lifted high and dry by tectonic forces. Once on land, rain and snowmelt filter down from the surface and gradually dissolve the limestone and carry it away. That has left some pretty amazing gaps in the ground.
A few caves have more extreme stories of excavation. Sometimes geology conspires to deliver a powerful dose of acid from sulfurous mineral deposits. That's the case at Lechuguilla Cave in southeastern New Mexico, where the mineral mixture carried by groundwater into the cave has produced yellow-tinged formations not of limestone but of delicate gypsum — a mineral made of sulfur, calcium and water.
Lechuguilla is noteworthy for another reason: It's huge. Since its discovery in 1986, more than 100 miles of continuous caves have been mapped, down to a depth of 1,567 feet. That makes it the fifth longest cave in the world, the third longest in the U.S. and the deepest limestone cave in the country.
Another cave with even greater doses of acid is Mexico's Villa Luz Cave. In Villa Luz sulfuric acid forms from rotten-egg-smelling hydrogen sulfide gas that bubbles up in springs. But acidity is no problem to the bacteria that have evolved to live in its depths. In fact, hydrogen sulfide gas provides chemical energy that feed bacteria — just like in deep-sea hydrothermal vent communities.
A far less corrosive, but completely outsized, relative of Villa Luz is in China's Guangxi and Chongquing provinces. The strange, mushroomy landscape of these parts is due entirely to a vast and monumental layer of regional limestone that is dissolving away. Hidden among the strange hills are remarkable sinkholes called tiankeng, or "sky holes," that are deep and wide enough to hide a few Empire State Buildings. These were all caves once, until their roofs collapsed. Today visitors can actually walk through caves beside an underground river to reach the bottom of one of these sheer-walled holes in the earth.
