I''m in a cave, lying on my side atop a bed of mud and sand washed in probably decades ago by some forgotten storm. Above me a pleasant fall afternoon warms a pastoral valley. Leaves have begun to turn beneath the spotless blue sky. Up there the Cumberland Plateau dominates the southern horizon, rearing like a green tsunami poised to crash over central Tennessee. In a sense the plateau is an ocean, if one long dead: its stacked sediments bristle with the fossilized remains of Carboniferous sea creatures. Beneath the plateau''s hard sandstone cap, voids riddle the softer limestone-more caves per square mile than in any other location in the United States, according to data compiled by the National Speleological Society. These water-carved caverns send out tendrils beneath the plateau''s edges. They can run on for miles, curling and coiling like a labyrinth Lovecraft might have imagined.

Several long caves reach this valley. I''m digging with hopes of breaking into a new one from a short, allegedly dead-end passage. The surrounding landscape is karst, a term derived from a German word describing the geology and topography of the Dinaric Alps, now applied to any limestone landscape featuring caves, sinkholes, and streams that vanish underground. I was sweating before I began this work because I could not find the entrance, hiking an unnecessary hour through boulders, chest-high thorns, and cow pies before giving up, driving up to the ridge for a cell signal, and calling the landowner for more detailed directions. Then I drove straight to it, thorns scratching at my rented SUV, boulders and stobs threatening to gouge the pan. I geared up and crawled over the rough cobbles of the entrance to begin digging in a smooth-walled rotunda at the end of the known space. Barely two hundred feet long, this little cave has been known to local residents for over a century. Proof is written on the white limestone ceiling in the form of a half-dozen blackened signatures and dates, scrawled in candle smoke by rural visitors during the late 1800s and early 1900s.

Elsewhere in this valley are caves explored by much earlier settlers from early in the nineteenth century all the way back to Native Americans who lit their way with cane torches thousands of years ago. People have always been drawn underground. The oldest human remains have been discovered in the caves of Africa, Europe, and Asia-some of them older than our most ancient common ancestor. A spate of studies of human mitochondrial DNA has pushed forward the date when a small band of African humans began to populate the rest of the world. As recently as sixty thousand years ago, or fewer than two thousand human generations, our common ancestors may have journeyed outward. Traces of their lives persist: caves serve as repositories for the earliest known examples of art, basketry, shoes, and clothing. Humanity''s more recent spread through the American South is also chronicled below ground. Less than an hour''s drive away from the spot where I lie digging, I once followed bare footprints in soft, damp cave mud, noting bits of ash and mineral samples dropped by three walkers.

Protected from casual obliteration by colored plastic flagging and extreme secrecy, these prints were made by the lined, leathered feet of explorers who traversed the passage more than four thousand years before me, according to the carbon dating of bits of river cane that fell from their flickering torches. A big-eared bat chirps nearby. Ruffling leathery wings with a drumming sound, he objects to my intrusion into his normally silent chamber. Over the past decade much of the bat population of the eastern United States has been decimated by white-nose syndrome, a deadly plague spread by fungal spores, but the few individuals I can see appear robust and healthy. Their annual hibernation period will soon approach. For now the weather outside remains warm enough that I know they will exit at sunset, each consuming hundreds of insects before returning at dawn. Except for bats and a few cave crickets, I''m alone in a passage perhaps twenty feet wide and twelve feet high at the center. The old signatures are spread over a comfortable alcove where the ceiling height is about six feet.

Farther from the center of the chamber, the roof slopes downward to meet the floor, giving the room the appearance of a lens. At the edge where I''ve worked for the past hour, the white limestone ceiling sits no more than twenty inches above the dried mud on which I recline. I reach with a garden hoe into a still smaller space, barely wider than the hoe''s blade: the spot where all present-day drainage vanishes. Water is rare here. The entrance sits on the high side of a sinkhole so that it only comes in during the largest floods. Following a list of clues over the past few months, I have found reason to believe that somewhere beyond the reach of my hoe unexplored passages and chambers await, perhaps connecting to a hidden borehole winding southward from the plateau. I can''t say that I''m here merely in the hope of scientific discovery. Other cavers far more accomplished than me have also been poking into sinks in this area, seeking a back door into a known subterranean system many miles long.

I deeply admire these explorers: I once wrote a glowing profile of their cantankerous patriarch, Marion O. Smith, in Sports Illustrated, a publication not normally known for caver stories. Smith is by far the world''s most experienced caver, with well-documented trips to over eight thousand separate caves in his seventy-seven years. Each of these caves is meticulous documented in a shelf of journals going back over sixty years, yet he calls the "most experienced" title "hogwash.".