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See the Ugly Beauty That Lives in a Toxic Cave

Norman Pace collects samples of a microbial mat.
Microbiologist Norman Pace collects a sample of a brainy-looking mat made of microbes (called a vermiculation) that coats the ceiling of Sulphur Cave.
Photograph by Norman R. Thompson

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.

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.”

A snottite found in a sulphur cave.
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.

Worms in Sulphur Cave, Steamboat Springs, Colorado. These worms are believed to live on the chemical energy in the sulfur in the cave, similar to the way tube worms live in a world without light at the bottom of the ocean. Also visible on the left side of the image are streamers—colonies of microorganism, similar to those seen in hot springs in Yellowstone National Park. Photograph by Norman R. Thompson
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.”


Correction: The cave is 180 feet long, not deep. This has been updated, and I  deleted a sentence about organic matter in the cave’s travertine—the cave’s sulfur comes mainly from geothermal activity, with microbial breakdown of organic matter as an additional, but minor, source. A clarifying sentence has been added. —EE (updated 6/15)

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How This Fish Survives in a Sea Cucumber’s Bum

Picture of a leopard sea cucumber excreting cuvierian organs, which look like white streamers, for defense
A leopard sea cucumber excretes its organs for defense. Photograph by WaterFrame, Alamy
Photograph by WaterFrame, Alamy

In 1975, Victor Benno Meyer-Rochow was diving off the Banda Islands in Indonesia, when he collected a leopard sea cucumber—a cylindrical relative of starfish and sea urchins. It was a large and stubby specimen, 40 centimetres long (16 inches) and 14 centimetres wide. He dropped it in a bucket of water, which he placed in a refrigerated room.

Sometime later, a slender, eel-like fish swam out of the sea cucumber’s anus.

It was a star pearlfish, and it wasn’t alone. Another wriggled out. And another. After ten hours, 14 pearlfish had evacuated the animal’s bum, each between 10 and 16 centimetres long. Another one stayed inside.

There are many species of pearlfish. Some live independently, but several make their homes in the bodies of shellfish, starfish, and other marine animals. Indeed, they got their name after one individual was found inside an oyster, dead and embedded within mother-of-pearl.

But sea cucumbers are their most infamous hosts. Having found one by following its smell, a pearlfish will dive into the anus headfirst, “propelling itself by violent strokes of the tail,” according to Eric Parmentier. If the sea cucumber objects and closes down its anus… well, it still has to breathe.

Oh yeah, sea cucumbers breathe through their anuses. By rhythmically expanding and contracting their bodies, they drive water through the anal opening and into a branching, lung-like structure called the respiratory tree. This process creates gentle currents that a pearlfish can use to find its hosts. It also creates a vulnerability, because a sea cucumber that’s clenching its butt is also holding its breath. When it exhales, as it eventually must, it dilates its anus, allowing the pearlfish to thread itself in. This time, it goes tail-first, bit by bit, breath by breath.

Some species just use the sea cucumbers as shelters. But the Encheliophis pearlfishes are full-blown parasites that devour their host’s gonads from within.

Pearlfish are typically found alone, and adults have been known to kill rivals that try to infiltrate the same host. Still, as Meyer-Rochow found, the fish can sometimes be more sociable—or at the very least, tolerant. No one knows why. It’s possible that when sea cucumbers are rare, the fish are forced to share a host. Alternatively, they could have gathered to breed. “If indeed the 15 fish entered for sexual reasons, one cannot help but think of the orgy that must have taken place inside the sea cucumber,” Meyer-Rochow says.

These anal abodes aren’t easy places to live.

These anal abodes shelter pearlfish from predators, but they aren’t easy places in which to live. Sea cucumbers produce bitter toxins called saponins that punch small holes in the membranes of cells. These chemicals ought to be especially destructive towards fish gills, whose cells come in thin, sensitive layers with a large surface area.

Pearlfish should be especially vulnerable, since they are literally swimming inside the saponin-producing structures. And yet, when Igor Eeckhaut exposed various fishes to sea cucumber saponins, the pearlfish survived 45 times longer than other species. How do they cope?

Sea cucumbers resist their own poisons because their cell membranes comprise special chemicals that interact less strongly with saponins. But Lola Brasseur from Eeckhaut’s team found that pearlfishes don’t use such fancy chemistry. Nor do they have any tricks for detoxifying the saponins.

Instead, they rely on mucus, which they secrete onto their skins. The mucus helps to lubricate them on their way into their hosts, but it also acts as a physical barrier against the toxic saponins. It’s especially effective because the pearlfishes make so much of it—six to ten times more than other fishes that have no interest in sea cucumber bums.

Of course, none of this explains why the sea cucumbers don’t use their most effective defence. When threatened, they can expel their respiratory trees at their attackers, relying on their regenerative powers to re-grow the lost organ.

Why, then, do they never evict their pearlfish lodgers in this way? No one knows.