Cave hunt.

underground secrets: get ready for the dark and dirty netherworld

Ready to plunge into a world so dark the sun hasn't hit it in millions of years? A place where bats snooze upside down in clusters of 1,000 or more and mineral deposits dye the walls crazy colors.

Then grab your helmet, flashlight, and knee pads. It's time to go spelunking, or cave exploring. The sunlight fades from view as you rappel, or descend on a rope, into utter darkness. "It's like entering a movie theater," says geologist Louise Hose of Westminster College in Fulton, Missouri. "Your eyes have to adjust."

Now sink down to your belly and squirm through a tunnel the width of a manhole cover until--finally!--you emerge inside a cavern the size of a school gymnasium.

The only sound: dripping water. The only light: the beam from your light. Put on a jacket; it's cold down here. Cave temperatures down to 305 meters (1,000 feet) match the average yearly temperature at a cave's surface. That's because caves are too deep to be affected by seasonal shifts in weather outside. Typically, cave temperatures hover between 24 [degrees] C (75 [degrees] F) and 1 [degrees] C (30 [degrees] F).

But keep going. As you descend below 305 meters, you'll be warmed by geothermal energy, heat generated within the Earth.

Welcome to the woolly world of caving, a sport long romanticized by adventurous teens and scientists, too. Now, a new generation of explorers is turning to the unexamined limits of caves that beckon just below the Earth's crust. Geologists estimate that less than half the world's caves have even been located. What secrets do they hold? Speleologists (cave scientists) are examining the bottom of known caves to identify the deepest caves on Earth. They are sifting debris inside caves to understand earthquakes. They're even studying the peculiar life underground to see if it holds clues to life on other planets!

the underground everest

Just as mountaineers aspire to reach the world's highest peaks, speleologists compete to see who can go deepest. "Everyone wants to break the record," says geologist Ernst Kastning of Radford University in Virginia. "Everyone wants to bag the deepest cave in the world."

At the moment, the deepest cave known is Lamprechtsofen-Vogelschacht (LAHM-prek-so-fen VO-gehl-shakt), which burrows 1,632 meters (5,354 feet) beneath the Austrian Alps. It's the second cave known to surpass a mile in depth.

However, speleologists now believe that Mexico's Sistema Cheve (sis-TEM-ma CHE-vee), discovered in the late 1980s, extends even deeper than the Lamprechtsofen-Vogelschacht. Like most super-deep caves, Sistema Cheve was created by carbonic acid, a corrosive liquid formed by rainwater and carbon dioxide. Over thousands of years, carbonic acid eats away at limestone, a type of rock formed from the skeletons and shells of tiny creatures.

Before officially declaring a cave the deepest, speleologists have to map its contours all the way to the bottom. The competing teams of speleologists at Cheve, like scientists working in caves everywhere, use three instruments: a magnetic compass for measuring declination, or direction; an inclinometer for downward slope; and a tape measure for distance. The cavers then use trigonometry, a math for calculating angles, to draft a three-dimensional map of the cave.

Descending for up to a week at a time, speleologists have charted Cheve to a depth of 1,386 m (4,547 ft). That's just 246 m (807 ft) short of the Alpine record. But now, all progress is blocked by a body of water, known as "Terminal Sump," that fills a portion of the cave. The only way to find out how far the water extends and what lies beyond it is to swim through it with a scuba tank. "Cave diving is extremely dangerous," says geologist Louise Hose. "The water reaches to the ceiling, so you can't swim to the surface. Your swim fins kick up silt so you can't see anything." About 25 people die each year scuba diving in caves.

But don't forget about the super-long as well as the super-wide. Mammoth Cave in Kentucky, the longest cave in the world, with a known running length of 563 kilometers (350 miles), may, in fact, stretch to as much as 804 km (500 mi). Scientists hope to nail down the truth before long!

rock eaters

A handful of NASA scientists are searching the dark crevices of a cave in New Mexico for clues to life on Mars.

Sound farfetched? NASA exobiologists (scientists who look for extraterrestrial life or life on other planets) have ruled out finding life on the surface of Mars. But they still think Martian life could lurk beneath the planet's surface. To learn more about what subterranean Mars life might be like, the exobiologists have teamed up with speleologists examining microscopic creatures living in eternal darkness in Earth's caves.

In the comers of Lechuguilla Cave (leh-cho-GHEE-ya) in Carlsbad Caverns National Park, New Mexico, speleologists have found a fragile ecosystem of microorganisms (tiny life forms including bacteria and protozoa) that can survive without sunlight. Unlike most caves, Lechuguilla contains little organic (biological) matter because it was sealed until recently. Speleologists have found that, amazingly, the microorganisms eat elements, or basic chemical substances, such as iron and sulfur found on cave walls.

Scientists believe the climates of Mars and Earth were similar 4 billion years ago. Microorganisms that developed on Earth could also

have lived on the surface of a warmer, wetter Mars. So what happened to that life? It might be subsisting on the sulfur-rich Martian soil underground. "If the surface of Mars grew colder and dryer," says NASA's Penny Boston, "the subsurface may be the last refuge of any life that lived there."

Scientists will know more when NASA's Mars Climate Orbiter collects information on the planet's geology later this year. "Eventually, we'll go to Mars ourselves to look for life," says Boston. "But in the meantime, life in caves will help us think about what to look for on that planet."

How a cave is formed

Rainwater and streams seep into limestone cracks. Along the way, water absorbs carbon dioxide to form an acid called carbonic acid.

Carbonic acid eats away, or dissolves, limestone. As water carries the debris away, the cracks grow wider and deeper.

Enlarged cracks admit more carbonic acid, which wears away more rock. Thousands of years later, the water may drain away, leaving caverns.

RELATED ARTICLE: cave quake

If an earthquake rocked your town, emergency crews would clean up the mess. But when earthquakes shake caves, the damage sits intact for thousands of years. Speleologists working in seismically active areas are now studying cave debris to learn more about paleo-seismicity, or ancient earthquakes.

The first clue speleologists look for: piles of stalactites (icicle-shaped calcium carbonate deposits) that have fallen to the cave floor, presumably after tremors dislodged them from the ceiling. They also study broken stalagmites, which rise from the cave floor.

Speleologists then assign a rough time frame to the quake by measuring the age of the fallen rock and the fresh formations that replaced them. How do they date ancient rocks? By using radioisotope dating, a technique that measures the gradual decline in radioactivity of the calcium carbonate deposits.

French geologist Eric Gilli recently proved that a dam in Turkey was built on the site of a massive earthquake sometime between 5,000 and 800 B.C.

Where else might unknown fault lines lie?