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                                                                                SCIENCE, Page 64

Subterranean Secrets


Though dark, dank and dangerous, caves are proving to be ideal
labs for learning about evolution, pollution and even hidden
oil

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, Flor