Caves have a subconscious hold on our imaginations - for our ancestors, they were not just shelter from the elements but also symbols of the womb, gateways to the underworld, places of wonder and mystery.
Not so mysterious today, but still full of wonder, caves and caverns continue to draw the adventurous, the curious and the scientific.
There are millions of caves around the world, on every continent, in every country. They are home to some of the strangest creatures on earth - eyeless spiders, hydrogen-eating bacteria, worms that glow and other organisms yet to be discovered.
Fred Luiszer, a cave scientist at the University of Colorado in Boulder, says that even space scientists are interested in life underground. ?If they find life on other moons and other planets, life will probably be very similar to what we?re finding in caves.?
In dark passageways which researchers work diligently to keep uncontaminated, scientists have discovered microbes that show promise as cancer fighters. But some cave life can be deadly.
A sulphur cave in Steamboat Springs, Colorado, is home to a rare form of gooey, dangling, toxic bacteria. Their colonies look like mucus and, according to Luiszer, they got their scientific name from the slang term for what drips out of your nose - snot.
?They are called snottites," he says. "I mean, when you look at one of them in the cave, it looks just like snot. I?m not kidding you.?
Snottites thrive on sulphur fumes, and excrete battery acid, so the cave is a hazard for the occasional amateur who ignores warning signs and ventures in.
?You pass out immediately, and if you stayed in that environment for probably, I?m guessing more than an hour or two, it would kill you,? says Luiszer.
So far, rescuers have saved the handful of people who have fainted in the sulphur cave.
Not all cave dwellers are microbial. In Colorado, scientists recently discovered a tiny red, blind, pseudo-scorpion. Bears also love caves and so do bats. In fact, cavers must be careful not to disturb bat colonies.
?Bats are hibernating creatures and if you wake them up in the wintertime," says Mark Masyln, a Colorado geologist and caving expert, "they go outside and their food source, insects, is not available and they die off. Which is why on commercial tours, you won?t see many bats.?
Caves with large bat colonies are closed to the public for another reason. A mysterious, deadly disease called white nose syndrome has killed more than 400,000 bats in the United States since 2006. Once a colony is infected, the U.S. Fish and Wildlife Service warns that often, over 90 percent of the bats die.
So far, the disease is concentrated in the northeastern U.S. To reduce its spread, wildlife experts have asked cavers to avoid caverns which are not already frequented by tourists or caving groups.
But scientists like Maslyn are trained to keep their gear uncontaminated by white nose syndrome and sometimes they?re permitted to go off the beaten path. Wearing boots and a caving helmet, he strides past the tour group in Colorado Springs' Cave of the Winds, and enters a hidden cave that he helped discover.
With a headlamp as his only light, Maslyn unseals an environmental door to reveal what he calls an easy entrance tunnel - half a meter wide - the size of a dinner platter. The reward for squeezing through is a muddy cave containing a dazzling, spiky crystal flower that?s taller than a man.
It?s a beaded anthodite bush. In decades past, Maslyn says, cavers used to carry anthodites away.
To protect these treasures, Masyln follows the caver?s motto: "Take nothing but pictures, leave nothing but footprints, kill nothing but time."
Culture of stewardship
That culture of stewardship is emphasized now, even at commercial caves, like Glenwood Caverns.
It starts even before a tour leader brings her group into the cave, when she warns them not to touch anything inside. As another protective measure, the tunnel leading into the caverns starts with a door that seals in the cave?s natural coolness and humidity and keeps out the hot, dry Colorado air. The guide opens the door for the group, and then shuts it behind them.
And a dozen meters down the tunnel, she leads them through another.
?To keep the water inside," she tells them. "That?s why we have so many doors.?
In some parts of the cave, the humidity tops 90 percent, making rock and mineral formations glisten. Some look like giant strips of bacon, giant soda straws, and popcorn. Stalactites hang high overhead, bathed in what has made them slowly grow over eons of time: water drops.
?If you?re going to be hit by a water droplet like that one, it?s a sign of good luck," the tour guide says. "And we call that the fairy kisses or the cave kisses. And you?re going to be lucky for the rest of the day.?
With luck, and stewardship, future generations will also enjoy the wonders of caves and the fairy kisses they have to offer.