Diana Northup steps into the jungle. She hikes for a few minutes before the smell hits her. She recognizes the rotten-egg stench instantly: hydrogen sulfide (H^sub 2^S), a potentially deadly gas. Unfazed, she descends concrete steps leading underground, right to the gas source -- Cueva de Villa Luz (coo-AY-vah day VEE-ah looz), a cave in southeastern Mexico.
At the base of the steps, Northup, a biologist from the University of New Mexico, splashes into a stream of murky water. It's full of the element sulfur. "It looks like somebody dumped a bunch of milk in it," she says. "You can't see your feet."
Northup creeps deeper into the cave, squeezing through narrow passages on her hands and knees. She keeps an eye on the gas monitor around her neck. In addition to hydrogen sulfide, levels of other hazardous gases like carbon monoxide (CO) can build in the cave. If the levels become unsafe, the monitor beeps. For extra security, a mask filters some of the toxic gases from the air she breathes.
Northup is braving harsh Villa Luz to study some slimy cave formations. Scientists think the cave may hold clues to what life on other planets might be like, if it exists.
Deep inside the cave, Northup finds slime dripping from the ceiling. These gooey cave deposits aren't stalactites (mineral deposits that hang from a cave's ceiling). In fact, they so resemble what leaks from a runny nose that scientists have dubbed them snottites. "If you blow on it, it wiggles and wobbles," says Louise Hose, a geologist and the director of New Mexico's National Cave and Karst Research Institute.
The goo is made by colonies of bacteria living on cave walls. But watch out: "What's dripping is sulfuric acid," she warns. With no sunlight to drive photosynthesis (process of capturing the sun's energy to turn carbon dioxide, or CO2, and water into food), these bacteria get their energy by combining the cave's hydrogen sulfide gas with oxygen. The waste product: sulfuric acid. "It doesn't burn [your skin] immediately," Hose explains. "But if you don't wash it off, it will create a second-degree burn."
Sulfuric acid doesn't just harm skin. Where it touches cave walls, the acid breaks down the limestone (rock made mostly of the mineral calcite) and carves out the cave. The acid reacts with the limestone and converts the rock to gypsum. This super-soft mineral dissolves easily in water and washes away. "It's pasty," says Hose. "When there gets to be enough of it, it just falls off the wall into the stream."
This type of speleogenesis (SPEE-lee-oh-JEN-uh-sis), or cave formation, is unusual. Most caves are etched out by carbonic acid. This chemical forms when groundwater (water that fills holes in soil and rocks beneath Earth's surface) picks up carbon dioxide from the soil. Like sulfuric acid, carbonic acid can weather (break down) limestone. Acidic water flows through the rock, hollowing out pockets that develop into caves.
But carbonic acid is much weaker than sulfuric acid. "It's the difference between pouring soda pop or pouring battery acid onto something that will dissolve," explains Hose. "The sulfuric acid is way more aggressive."
That means that sulfur-formed caves expand much faster than other caves. "[Villa Luz] is growing at an alarmingly fast rate," says Hose.
There are other sulfur caves around the world, including New Mexico's Carlsbad Cavern -- which has one of the largest underground chambers (open spaces) in the world. Still, no cave compares to Villa Luz. "The concentrations of hydrogen sulfide in Villa Luz are much higher than any other system that we know about in the world," says Annette Summers Engel, a geologist at Louisiana State University.
The gas seeps up through holes in Villa Luz's floor. Scientists suspect the hydrogen sulfide may be leaking through underground channels from a volcano that lies about 56 kilometers (35 miles) away.
Surprisingly, Villa Luz's hostile conditions haven't kept animals away. The cave's inhabitants include spiders, freshwater eels, and bats. Mollies -- inch-long, pink fish that eat the slimy bacteria found in Villa Luz's stream -- are another resident. "[The mollies] look like they're wearing turquoise eye shadow," says Northup.
Believe it or not, bacteria that have adapted to Villa Luz's harsh environment could hold clues about life that may have existed on other planets. There's evidence of caves on Mars, and some scientists think alien life may have started underground. "Conditions are very harsh on the [Martian] surface," says Northup. "Caves provide [a protected environment] because they are fairly stable in temperature." The recent exploration of Mars by NASA's rovers, Spirit and Opportunity, has also revealed that the planet's surface has chemical similarities to Villa Luz. "It was a salty, sulfur-rich environment, just like these caves," says Engel. Scientists think microorganisms similar to Villa Luz's bacteria may have formed in these Martian conditions.
That's why researchers keep returning to Villa Luz. By uncovering more clues hidden in the dark, they may one day help NASA scientists discover alien life. As Northup explains, "Every time you look, you find new things."