Subterranean Secrets

By CHRISTINE GORMAN CARLSBAD

THE POOL SITS SO STILL AND CLEAR THAT it remains utterly invisible unless glimpsed from an angle. Suspended overhead, cream-colored puffs of rock billow within arm's reach, seeming to defy gravity. Welcome to Lake of the Clouds, an enchanted spot of earth that has never seen the sun or felt the morning dew. Carved out of solid rock nearly 1 million years ago, this bewitching chamber lies 300 m (1,000 ft.) below the floor of the New Mexican desert at the lowest point in Carlsbad Cavern.

Getting there requires the skills of a subterranean mountain climber, which is why Lake of the Clouds is off limits to the public. The underground trek involves scrambling through narrow passages, navigating around steep crevasses and using ropes to descend two drop-offs -- the second of which encompasses a 60-m (200 ft.) cliff. Turn off the miner's light on your helmet, and you cannot see your hand in front of your face.

For a small group of seasoned spelunkers, hardy souls who love squirming through tight spots and tromping through mud, such a venture is as pleasant and relaxing as a Sunday-afternoon jaunt. Nor are the trekkers hindered by the surveying instruments, acidity meter and other tools they lug along the way. Led by geologists Art and Peg Palmer, these scientific adventurers are trying to determine what the evolution of the cavern can tell them about prehistoric climates, the ecological health of the surrounding region -- even the likelihood of finding oil in limestone deposits around the world.

In the process, the Palmers and their colleagues are transforming speleology -- the study of caves -- from an oddball hobby into an extraordinarily fruitful field of scientific investigation. Old views of caves as static places untouched by time or weather have been shoved aside. Replacing them is a growing understanding of the complicated ways in which caves interact with the land above and around them. "Wondering where a cave goes, what is down there and how it formed is really the essence of science," says Art Palmer, who is a professor of hydrology at the State University of New York at Oneonta. "Yet most of us were too excited about exploring to realize this when we were first starting out years ago."

Caves can be pounded into existence by ocean waves, plowed open by ice or formed by lava. But to speleologists, the most interesting are those that have been etched out of limestone by acidic water flowing underground. For a long time, researchers believed that nature could accomplish this feat in only one way: through the action of carbonic acid, which is produced when water reacts with carbon dioxide. The weak acid slowly dissolves bedrock. An underground stream forms, and an elaborate network of chambers like those found at Mammoth Cave in Kentucky takes shape. The unusual limestone terrains where this process occurs are known as karst, named for one such region in Slovenia that is famous for its caves. About 15% of the earth's terrain is karst. By studying and dating the old subterranean waterways, researchers can tell how wet or dry past climates have been.

As a newly carved cave fills with air, drops of water seep in through the walls and ceiling. Minerals trapped in these trickles begin to precipitate out of solution, hardening into a stunning array of underground ornaments. Stalactites drip down from the ceiling. Stalagmites creep up from the floor. Miniature forests made of twisted branches of calcium carbonate stretch out from the walls. Many of the formations are so delicate that they can easily be destroyed by the presence of humans.

However, this scenario never really fit one celebrated site -- Carlsbad Cavern. "At Mammoth Cave, you can follow the path of the water from beginning to end -- just like some kind of elaborate plumbing system," says Carol Hill, who works with the University of New Mexico and is a legendary figure in cave science. "But you can't do that for Carlsbad. The cave keeps stopping where it shouldn't."

Hill helped clear up the confusion in the 1980s by carefully measuring the sulfur content of samples taken from the caverns. Her work proved that Carlsbad was carved not by carbonic acid but by sulfuric acid, produced by a reaction between oxygen dissolved in groundwater and hydrogen sulfide bubbling up from deep below the earth's surface. This highly toxic solution, which would have killed anyone present at the time, sculpted the many subterranean chambers at Carlsbad.

Even more to their astonishment, researchers discovered that biology played an important role in the rock-dissolving process. By poring over slices of limestone under microscopes, scientists found the fossil remains of primitive bacteria that had thrived in the once hostile environment. Using sulfur instead of sunlight as their source of energy, these organisms actually bolstered the acid's power to etch rock. Descendants of these strange microbes have recently been found and are being studied at Lechuguilla Cave, not far from Carlsbad.

By piecing together the sulfurous origins of Carlsbad and other caves, speleologists have done more than satisfy scientific curiosity. They have also laid the foundation for some promising new ideas in oil exploration. Hydrogen sulfide, which is sometimes emitted as buried organic material decomposes, often appears in petroleum fields. Core samples of rock produced during drilling suggest that some oil and gas deposits are trapped within ancient cave systems that formed hundreds of millions of years ago. "So, about five years ago, some of us started looking in modern caves to see what they could tell us about where to hunt for oil," says Robert Handford, principal geologist at ARCO's research center in Plano, Texas. "It's been a truly eye- opening experience that has made us interpret some of the cores we bring up in a completely different manner." Because of the link between oil and caves, ARCO is starting to use remote-imaging technology to detect the presence of underground caverns. "My guess is that we will be able to find significant amounts of oil and gas this way," says Handford.

Researchers are also applying what they have learned from caves that, unlike Carlsbad, are still actively growing. Among those lessons are some alarming insights into the way industrial contaminants spread underground. In most parts of the U.S., the ground is solid and compact and water flows down through it at a rate of less than 30 m (100 ft.) a year. But about 20% of the U.S.'s fresh water flows through the myriad cavities and pores of limestone karst, often traveling 1 km (0.6 mile) overnight, taking unpredictable turns and sometimes bubbling up to the surface through a spring. Containment of a toxic spill in such terrain is virtually impossible. Even ordinary garbage that is dumped in a sinkhole can contaminate groundwater miles away.

The potential for disaster is only beginning to be appreciated. For years residents and businesses around Bowling Green, Kentucky, pumped or buried solvents and wastes in the ground, heedless of the fact that the city of 40,000 sits on karst. In effect, they turned the underlying caves into a toxic sewer. Twice during the 1980s, benzene and other chemicals rose up from the caves into homes and elementary schools, endangering people's lives.

Fortunately, speleologists at Western Kentucky University were able to use their knowledge of how water flows through caves to trace the source of the fumes and put a stop to the contamination. They plan to map out more of the underground caves and passageways in order to better understand which areas are at highest risk. Communities built on karst in Tennessee, West Virginia, Florida and Missouri may someday follow suit.

These efforts at prevention will not eliminate accidents, however. "One of the biggest fears I have now is highway and railroad spills," says Nicholas Crawford, director of the Center for Cave and Karst Studies at Western Kentucky. Two years ago, a freight train carrying hazardous chloroform jumped the tracks near Lewisburg, Tennessee. "If that train had derailed in Bowling Green, it would have been a catastrophe," Crawford says.

Caves have led to new insights about evolution. The absence of light and scarcity of food limit the number of species that can survive underground. Most common are crickets, beetles and eyeless fish. "We see simple communities that may be made up of only four species," says Tom Poulson, professor of biology at the University of Illinois at Chicago. "But that allows us to look in greater detail at what is going on, say, between predator / and prey." As a result, biologists can study subterranean ecosystems in their entirety -- a feat that often proves impossibly complex aboveground.

By looking at fossil specimens and studying current species, researchers have concluded that most cave dwellers started out at the entrance of the cave. As they and their descendants traveled deeper inside and away from sunlight, they began to lose their eyes and develop other sensory organs to compensate. But is this loss an active process or just a question of disuse? "That's been a raging debate ever since Darwin's time," Poulson says. "What we've found is that it's disuse. There is no natural selection to screen out any bad mutations that affect the eyes. So eventually they disappear."

Keeping their own eyes open as they peer into the inky depths, researchers are finding that caves reveal much about the world above and around them. "Science has overlooked caves for far too long," Art Palmer argues. "And yet caves tell us a lot about the recent history of the earth, about ecology, even about things that are economically important." As one of the few sciences that is also a sport, speleology is finally getting the respect and attention it deserves.