The descent begins with an insignificant looking gully, a dry rut that slices into the cattle-trampled chaparral along the Utah-Arizona border. The gully turns down slowly but steadily, first six-feet deep, then 12, then 20 and 30. The earthen walls give way to sandstone; soon we're walking on bedrock carpeted by pebbles and sand. Fifty feet down, 80, 100: The gorge remains narrow, barely 10 feet across, the sky now a skinny strip of blue high above. And it just keeps going, diving into the dim gut of the underworld.

It's cool as a cave down here, and the air smells of stone, water, Time. The rock that surrounds us is at least 200 million years old, and the elements have scoured its surface to form the pockets of sand beneath our feet. We're wading through Time, descending Time, breathing Time. The drab, sun-bleached desert surface we left behind a half-hour ago already seems far away, in another world, a separate reality.

A slot is a canyon compressed into surreal proportions, 300 or 400 feet deep, with sheer vertical or even overhanging walls and a floor so narrow you have to edge along single file or even sideways to get through. Think of a canyon that feels like a tunnel, a cave, a sepulchre, a tomb. You'll know one when you're in it.

The Colorado River Plateau, the high stone desert that sprawls across parts of Utah, Arizona, New Mexico and Colorado, is imbued with a palpable sense of mystery, of shrouded secrets. Out in those intricately folded origamis of bedrock and sand there are mesas that have never been scaled, hanging slickrock gardens no one has explored, uncharted cliff dwellings hidden on shadowy ledges. But nowhere is the sense of the unknown so powerful as in the slot canyons: Coyote, Spider, Buckskin, Cheesebox, Antelope and the rest.

We're at least 200 feet deep now, maybe more. Looking up, we see a reef of driftwood, logs the size of cannons, jammed between the rock walls 50 feet above our heads. Wouldn't want to be down here in a thunderstorm, no way. In August 1997, 12 tourists were hiking down Lower Antelope Canyon, outside Page, Ariz. A heavy downpour hit upstream. Ten minutes later, a 40-foot wall of water slammed through the narrow passage. All but one drowned.

There's just no easy way out of these slots: The unrelenting walls go on for miles, smooth, without a break. And a storm doesn't even have to make a direct hit on the canyon to trigger a killer flood. These canyons are surrounded by hundreds of square miles of bare rock and hard soil that funnel every raindrop to the lowest point. A 10-minute cloudburst 10 miles away can put something carrying the force of a tidal wave right where you're walking. Caveat emptor, pilgrim.

In the late afternoon, we find a place to camp: The canyon expands into an amphitheater a hundred feet long by a hundred feet across, and there's an islet of clean white alluvial sand the size of a tennis court against the left-hand cliff, its top maybe 20 feet above the canyon floor. Not much help if a flash flood sends water crashing down, but good enough for the night. It's not only a nice campsite, it's the only possible campsite we've seen since we entered this morning.

We pitch our tents, fire up the stove, cook up tea and Thai noodles with dried shrimp, and break out some brownies. There's still a bright ribbon of sunlight along the rimrock, but down here it's all deepening shadow: muted gold, mauve, brooding gray. An unseen hawk cries somewhere up above, a harsh, defiant kraa-kraa! For a moment the invisible bird's elongated shadow flies like a boomerang across that high stripe of sun, then vanishes as the hawk soars away.

Bats dive and dart through the dusk. A frog croaks from a secret pool somewhere down-canyon. Brackety-brack, ko-ax, ko-ax!

Our little island of headlamps and flashlight is engulfed by dark. The distant ribbon of sky turns to stars as if somebody flicked a switch, but the light doesn't penetrate down here.

Is there anything better than waking up in a lost canyon? Lie in the sleeping bag, half conscious, half dreaming, and listen to the John Cage music of the underworld. Darkness slowly, imperceptibly melds into the soft gray light of dawn. And then, somebody else is awake out there. A match struck, the whoosh of a stove flame, water burbling into a pot, and then the pungent, spiky smell of unleashed coffee grounds, like a djinn unraveling from a lamp . . .

By 8:30 we're packed up and hiking, descending farther, deeper, into ever more awesome zones. In places, the canyon is so narrow we can barely squeeze through with our packs. The cliffs rise in delicate scallops and waves, translucent as seashell at the edges. The floor of the canyon is paved with smooth pebbles and cobblestones, webbed with rippled sand. I've seen the best Zen gardens in Japan, and they have nothing on this place. Nature's accidents beat man's art, four out of five.

In midmorning we round a curve in the canyon and come upon a pool. A really big pool. It stretches as far as we can see, 200 feet at least, to the next turn in the canyon. More important, it's deep: first knee-deep, then thigh-deep, then waist-deep. And it's cold, painful, teeth-clenching cold. We forge on.

The pool goes on for the next mile. In places, it's nearly chest-deep. In other places, it's clogged by tangled reefs of driftwood you have to climb under, over, through. Hard work, lugging 40-, 50-pound packs through this amphibious realm. And we're lucky the water isn't deeper: Some years, we find out later, this pool is too deep to walk through. You have to bring along an air mattress, float your pack on it and dog paddle, pushing your gear before you like a tugboat.

But this, after all, is why we came: because this is a place most people won't go. Space is the ultimate travelers' luxury these days, a luxury that here can be purchased only with sweat equity, not cash. We wouldn't be anyplace else on Earth.

For more information on slot canyons in the United States, check out GORP (Great Outdoors Recreation Pages) on the Web at www.gorp.com.

Rob Schultheis is a writer in Colorado.