It's hard enough for little critters to get to the depths of the pristine cave hidden in a nook on this North Georgia mountain. But for humans, cramming through the claustrophobic crack at its entrance is a contortionist act.

"You have to fold like a lawn chair to get down here," said Carlos Camp, a biology professor at Piedmont College in Demorest, Ga.

The only visitors who brave the twists and turns of this cave are hard-core adventurers and scientists such as Camp, who has been coming here for six years to count the salamanders that call the dark, rocky crag home.

As more hikers trudge through the nation's caves, pristine caves are getting rarer; footprints, trash and even the oil on the hands of visitors endanger the caves' fragile ecosystems.

Chemicals from garbage and discarded batteries, and pesticides from high above can seep into water tables that lie along the type of land that forms caves and sinkholes, where 25 percent of the nation's drinking water originates. Timber harvesting and farming can also force sediment into the caves below, clogging nooks and crannies and jeopardizing the animals that hang on a delicate thread.

Deep underground, where animals depend on scarce energy sources and often live far longer than their surface-dwelling cousins, the death of even a single bat could doom a cave's ecosystem.

"The way to protect caves is don't go in them," said Allen Padgett, a retired wildlife technician who captains a cave rescue team out of Georgia. "But then to protect caves, we have to understand them and know where they are."

Padgett has surveyed thousands of caves in search of elusive "virgin" caves where no one has ever ventured. He endures 24-hour hikes deep into uncharted caverns, so vulnerable that even the sound of the human voice can be damaging. In one cave, the vibrations from talking shattered needle-thin crystals that took an eon to form.

"That is how fragile parts of this world are," Padgett said. "That place had never, ever heard a human voice. The rain doesn't fall there and the sun doesn't shine -- that place hasn't been touched in millions of years."

To safeguard pristine caves, explorers and scientists keep their locations close to the vest. In Georgia, the sites of many treasured caves are known only to a handful of cave aficionados -- and even the state's open records law protects them from being pinpointed.

In that vein, Camp and state biologist John B. Jensen asked that the locations of the little-known caves visited for this article be kept secret.

To get to one cave, the scientists carefully balance across a fallen log in the back yard of a pastoral farm, hopping off at a cave's waterlogged entrance. Bone-chilling droplets splash as they plod through the knee-deep water, probing the secluded chambers with headlamps in search of tiny feet or the hint of a tail.

In another cavern, Camp ducks beneath uncomfortably pointy crags, aiming the flashlight roped around his neck into nooks in the cave wall. He and Jensen spot a rare target -- the Pigeon Mountain salamander, which exists only in a 6-mile zone on one side of the northwest Georgia peak.

Nearby, yet a world apart from the underground unknowns that these explorers seek, the Pettyjohns cave lurks as a warning.

One of the most popular caves in the Southeast, Pettyjohns is packed on the weekends.

The opening is smooth, like a flume, sanded down by the thousands of scout groups and adventurers who visit each year. Once inside, the cave opens into an enormous cavern, marred by fading spray-painted graffiti, scattered litter and the smoldering remains of an illegal fire pit -- banned in the cave for safety and aesthetic concerns.

A club of cave enthusiasts maintains it, but even their persistence cannot keep the site in good condition.

"Look at Pettyjohns," Padgett says with a sigh. "If the world knew of the other caves like that, what would happen?"

Georgia state biologist John B. Jensen exits a cave. He and other experts work to see that cave locations are kept under wraps to protect them.