Lurking below the quaint ski town of Steamboat Springs, Colorado, lies a cave belching deadly gases. Its ceiling is dotted with snottites, dangling blobs that look like thick mucus and drip sulfuric acid strong enough to burn holes through T-shirts. And the whole place is covered in slime.

So why would anyone want to go there?

“Being in the cave reminded me of being inside a huge organism—as if I had been swallowed by some gigantic, alien monster from deep in the ocean or from outer space,” says photographer Norman Thompson.

Thompson joined a small group of scientists who are among the few people to ever explore Sulphur Cave, and who found it eerily beautiful, and brimming with strange life. As shown in National Geographic’s exclusive video below, along with spiders and insects, the cave holds sulfur-breathing microbes and a new species of blood-red worm.

New Discovery: Blood-Red Worms That Thrive in a Toxic Cave (EXCLUSIVE VIDEO)

EXCLUSIVE VIDEO: Clumps of newly discovered blood-red worms thrive in Sulfur Cave, which contains levels of toxic gases so lethal that any human who enters unprotected could quickly die.

“In a sense, we really were inside of an organism,” Thompson says, “or perhaps more accurately, an ecosystem. Because the cave is a colony of organisms, living together in a lightless ecosystem, powered not by sunlight, but by the sulfur coming from deep within the Earth.”

Inside the Belly

To enter the 180-foot-long (54 meters) cave, the intrepid scientists had to squeeze into a pit entrance, a hole in the ground that skiers might glide right past. And if you happen to visit without special equipment, you ought to glide past. Otherwise, the cave’s gases could knock you unconscious in a jiffy.

“It’s sort of foreboding,” says David Steinmann, a cave biologist at the Denver Museum of Nature and Science. “You have to climb and crawl down a wet muddy slop that’s stinky and smells like rotten eggs.”

View Images Snottites are thick, mucus-like blobs formed by bacteria growing in a sulfur cave. Photograph by Norman R. Thompson

“It’s belching toxic gases,” Steinmann says, “and in the winter you can see steam coming out. You have to stoop down and squeeze through to get into the first room. Once you’re in there, it’s totally dark.”

But when the team brought in lights, they found that the cave is also lovely, in its own way. Crystals made of gypsum glitter on walls, and a small stream washes across the floor. Long tendrils made of more microbial colonies wave in the water’s flow.

Thompson photographed the cave twice, entering only after scientists had aired out the crevice using large fans—appropriately, the kind normally used to flush out underground sewers. “Even with the poisonous air flushed out by the fan, the cave still stunk of sulfur,” he says.

Such sulfur-filled caves are rare, with some found in Mexico and Italy. The high levels of sulfur that create the gas in Colorado’s Sulphur Cave come from deep within the earth. The cave is formed in travertine, a type of stone formed by deposits from streams and mineral springs.

Hydrogen sulfide gas, which gives the cave its rotten-egg smell, can be deadly at high concentrations. Yet life thrives inside the cave despite both the hydrogen sulfide and carbon dioxide up to four times levels that could kill a human.

Wormy Wonders

The biggest surprise was the blood-red worms found in the cave. “There’s a hell of a lot of worms in there!” says Norm Pace, emeritus professor of microbiology at the University of Colorado Boulder.

View Images These worms in Colorado’s Sulphur Cave are believed to live on the chemical energy in the sulfur in the cave, similar to deep-ocean tube worms. On the left are streamers—colonies of microorganisms similar to those in hot springs in Yellowstone National Park. Photograph by Norman R. Thompson

The small worms live clumped together on the cave floor, where they’re probably making a living by grazing on the bacteria growing in wet spots, Pace says.

They’re also intensely red, much like the famous Riftia worms found at deep-sea vents, which are also rich in hydrogen sulfide. Pace has studied life in the vents and expected the cave ecosystem to be similar. It wasn’t, exactly. The ocean worms have special structures called trophosomes filled with bacteria that are able to live on hydrogen sulfide; essentially they “breathe” it. The worms rely on the bacteria to do this, so Pace was surprised that so far, the team hasn’t found a special home for bacteria inside the Sulphur Cave worms.

As for the cave worms’ bright red color, it probably comes from high levels of hemoglobin and related compounds that protect the worm from hydrogen sulfide. Steinmann and his colleagues described the worms this year in the journal Zootaxa.

They named it Limnodrilus sulphurensis, in honor of the sulfur that powers the base of the food chain in this otherwise deadly environment.

“It took over a year for the sulfur smell to gradually air out from my cave coveralls,” Thompson says. But would he go back? He’s still drawn by its strange beauty he says, so yes— “in a heartbeat.”