People who spend lots of time on rivers agree that these coursing bodies of water have spirits. They are alive. Respect them, and rivers will grant you safe passage over rough rapids and through narrow canyons. Disrespect them and beware. It is almost a given, for example, that any canoeist who brags he's never flipped his boat on a certain cataract will, the very next time, flip his boat there.

So it's uncertain what terrible thing I--or my guides--did to upset one of the wildest, most beautiful river canyons you've never heard of: Santa Elena Canyon deep inside Big Bend National Park along Texas's Rio Grande. Who knows why that freak event happened when it did, the way it did--an act of God that almost killed us by means few people will believe.

But it happened. And there are several witnesses. And maybe it's not so amazing after all. For if there exists anywhere on Earth a place big enough and wild enough and empty enough to harbor forces completely beyond our comprehension, it is here in the Chihuahuan Desert of western Texas, an expanse of sand, canyons and mountains the size of Maryland with a human population of just 13,000 lonely souls. Big Bend National Park forms the beating heart of that boundless desert world with 1,200 square miles of scrub bushes and roadrunners and rattlesnakes hard by the Mexican border along an eponymous southerly dip of the Rio Grande.

Here stand the "Phantom" Mountains, the final wilderness bastion of the fight-to-the-end Apache Indians. Their ghosts still roam free amid mountain lions and wild horses and black bears across desert peaks soaring to almost 8,000 feet. On the surrounding desert floor, stretching in all directions, weird shadows and blinding sunlight play tricks with your eyes until sunsets of lavender-rose incandescence finally break your heart.

Though endearing to the human imagination, this desert is hostile to humans in every other way, safeguarding its secrets with no fewer than 11 species of stinging scorpions, a plethora of tarantulas, precious little water and a labyrinth of cactuses whose thorns range in name from "horse crippler" to "eagle's claw." No wonder so few people have ever even heard of the desert rat here that never drinks water, or the remarkably wind-carved Indian faces on desert buttes, or the mysterious clouds of white light appearing late at night, over and over again, on the desert floor, with no explanation whatsoever.

Yet across this ghostly desert landscape where normal laws simply don't apply, the sanctum sanctorum of oddity is surely that appalling fracture in the Earth called Santa Elena Canyon. Between its 1,500-foot sheer cliff walls, the world has no horizon and water flows uphill and river rafts miraculously slip through rock doorways narrower than the rafts themselves. The Rio Grande here took 100 million years to slice through an entire range of mountains called today the Mesa of Eels, and the river twists and bends just like the slithering namesake. Just downstream, rumors fly among gringos and Mexicans alike of lost conquistador burial caves, where mad-smiling skeletons lay supine in medieval helmets and full breast plates, gripping swords with bleached, dusty knuckles that once killed and conquered.

It is here, in the middle of Santa Elena Canyon, in a 14-foot raft named Sun Dog, that the strange event in question happened. A mile into this giant vault of limestone, just above a series of challenging rapids, we heard the gunshots. They exploded only a few feet away, shattering our ears, sending us ducking for cover and scanning the canyon rim for deadly snipers. But it wasn't guns making that deafening noise that soon had us rowing madly, white as apparitions, mumbling prayers that wafted up the canyon walls.

The assault on your senses begins the moment you leave the airport outside Odessa, Tex., and point yourself south toward the Mexican border, four hours away. There's no fast way to get to Big Bend National Park. It's not near anything or on the way to anything.

And getting there means following two-lane gun-barrel roads that take you through a wonderful landscape of true-life cliches: tiny Texas ranch towns full of cowboy-hatted men and squeaking windmill wells, where the only traffic signal is a tilting yellow sign downtown saying, "Loose Cattle." In the distance you can literally see a hundred miles to buttes and mesas straight out of roadrunner cartoonland. The roadrunners themselves are everywhere, scampering across the hot asphalt as you pull over to a roadside burrito shack with Mexican tunes spilling from your car radio. The only thing moving between these infrequent towns are dust devils and lost burros and the occasional U.S. Border Patrol vehicle with giant whip antennas and all-terrain tires.

A lonely park ranger collects my fee at Big Bend's northern entrance around 3 p.m., and for a long time I see no one else inside. I pull over, walk a quarter mile into the desert and stop amid scattered creosote bush, prickly pear cactus, towering yucca and a violently beautiful arroyo carved by years of flash floods. There is no sound except the trill of a desert cicada and an occasional riffle of wind.

Astronauts have used the dry, rocky terrain of Big Bend to simulate moonscapes. But it's a moonscape brimming with life. Here on the desert floor I see darting lizards and the pink flash of a western coachwhip snake. And though nothing else moves across this ground that can reach summertime temperatures of 150 degrees, I know just beyond sight sit jackrabbits hiding in the shade of cactuses, their megaphone ears serving as complex air conditioners. In burrows below my feet lie kangaroo rats that never drink water, getting their moisture entirely from food, and expelling a bizarre paste instead of urine.

I return to my car, sidestepping a fist-size tarantula, then follow a wending road up into the Chisos Mountains. The name translates from Apache as "phantom" or "ghost," and the shadowy sunset light indeed gives the mountains a haunted feel. I stop to watch three black bears, extremely rare here, scramble up a rocky slope. A ranger later tells me he's worked in the park for a year and never seen a bear. I'm here an hour and I see three. Odd. Unsettling.

At first light the next morning, after a night in a quiet mountain lodge, I take a five-mile hike to the rocky summit of Emory Peak, the highest of these mountains at 7,825 feet. Here the air is dramatically cooler and the land greener with pinyon pines and scrub oaks. I pass pig- like javelinas and Sierra del Carmen white-tail deer along the way, but no people. At the summit, all by myself in a torrent of sunlight, I look down on a miraculous scene: a continuous sheet of clouds blankets the desert floor all the way to distant mountains on the Mexican side. I've ascended to Heaven.

After a while, the clouds part to the south like curtains, and there it is: the eastern entrance to Santa Elena Canyon. Even from 17 miles away, those quarter-mile-tall walls look freakishly high and forbidding against the rolling desert floor and the rippling current of the Rio Grande. The rest of the canyon is beyond sight, its body swallowed completely by the earth.

The next morning I meet my river guides on the north bank of the Rio Grande. Head guide Jim Haney and first mate Jason Hodgman quickly settle me and four other passengers--married couples from Georgia and Wisconsin--into two 16-foot inflatable rafts loaded with gear. We launch into the slow current, leaving behind a group of barefoot migrant workers asleep on the Mexican bank.

The cloudy Rio Grande is nothing spectacular here, barely three feet deep and maybe 100 feet wide. It's pleasantly fringed by river cane and brilliantly green salt cedars, beyond which the brown dryness of desert resumes, sprinkled with yucca and cactuses. We're soon officially back inside Big Bend park, which spreads to the horizon to our left while all of Mexico lies to our right. The next trace of human civilization will be a rutted dirt road we'll encounter two days from now.

Jim, deeply tanned behind wraparound sunglasses, is a river fanatic, guiding trips in Washington state in the summer and here in the autumn and winter. He camps out maybe 200 nights a year, he says, and his hobby is rafting Class 6 rapids--water so wild no one's successfully run it before. As a result, he's been repeatedly "Maytagged," which is to say he's been caught in underwater spin cycles for dangerously long periods before being spit to the surface.

By late morning, exactly as planned, we've fallen off the map, floating through trackless desert in the middle of nothing. The ghostly Chisos Mountains rise to our left, while here and there, wild horses appear atop nearby mesas, haughty and beautiful, rearing on hind legs.

Hours later we see in the distance a chimneylike butte called the Sentinel, whose wind-sculpted Indian faces keep stern watch over the river valley. One Indian wears a feather headdress, the other a wide headband around long hair, both faces formed by the vagaries of erosion and crumbling rock. It's hypnotizing. Beautiful. Unsettling.

Here Jim tells me about the "Marfa Lights," those queer clouds of white luminescence appearing frequently and with incomprehensible characteristics upriver on the Texas side. He has seen them often. He once saw a TV documentary where men with walkie-talkies approached a canyon where the lights had appeared. One set of men went down to the canyon floor to get a closer look, but then radioed back that the lights were gone, vanished completely. "What do you mean?" radioed the men still on the canyon rim. "You're right under the lights. We see you and the lights. You can reach up and touch them!"

It's no wonder people around here believe the Rio Grande, too, has its own strange spirit, one it safeguards by mysterious means. Take the menacing tree that tried to block our way, for example. It was late morning when Jim brought us around a bend and we saw the large salt cedar tree that had fallen directly over the only navigable channel of the river, with barely three feet of clearance below the tree's trunk.

"This," Jim said with a laugh that was part humor and real concern, "is the Tree of Death. We've all got to squeeze against the raft bottom and somehow float under that tree."

Just that morning, two other guides had canoed down to saw off the Tree of Death's most threatening branches. The river gods had chosen carefully, for salt cedar wood is so hard the men had to use a hack saw to cut it. Somehow we managed to float under the stubborn obstruction, the tree trunk scratching against our backs and grabbing at our hair as we crouched low to the raft floor.

Before sunset, an enormously tall and broad rock wall looms into view, seemingly blocking our way. It's the Mesa de Anguila, a k a the Mesa of Eels, and it's cleaved right down the middle by the towering, doorlike entrance to Santa Elena Canyon. We've arrived.

White-throated swifts dive for insects along the canyon rims as we set up camp on a rocky beach just outside the entrance, dwarfed by the intimidating beauty of 500-foot cliffs. Sentinel Butte is still visible in the far distance, the chiseled Indian faces watching us like spies as a violently beautiful sunset lights the cactuses on fire and turns the river deep purple and makes the sky a slow-exposure eruption of tangerine flames.

Jason, the guide whose raft I've been assigned to tomorrow, tells me over dinner that the only difficult rapids inside the canyon are at a spot ominously called the Rock Slide. Here massive fallen boulders create a bottleneck for rafts. But Jason quickly assures me that the danger of falling rocks is absolutely zero. Indeed, neither he nor Jim, with all the years riding on rivers, has ever seen a single rock fall from a mountainside or canyon into a river. Never. Anywhere. Falling rocks are such a rare and random occurrence that your chances of observing one are less than those of being attacked by a mountain lion.

We sleep that night under the stars, by the murmuring river. The sky is freakishly clear and the stars as dense as ice crystals spread across the universe's bedroom window. I lie on my sleeping bag, with Santa Elena Canyon just feet away waiting to receive us. I gaze up at the Milky Way, that long, cloudy band across the sky, and I sense myself as never before on the outer fringe of that fantastic galactic Frisbee, floating on a speck of dust, peering deep into the vast, faraway, unknowable core.

Soon after you enter Santa Elena Canyon the water starts running uphill. You pass through the canyon's monumental rock door, then drift around a sharp bend, then find yourself entombed by 70-story-tall limestone cliffs as far as you can see. Then, up ahead, the river starts flowing briskly up a steep incline.

It's an optical illusion, of course. Tectonic forces lifted the stratified bedrock here 100 million years ago and pushed it up along a fault line. The river then sliced the earth in half, exposing massive rock strata that appear to be perfectly level lines but in fact are tilting downhill toward your oncoming raft, making the river appear to flow uphill. Steeply uphill.

This miraculous image, though, has competition for your attention. For far overhead, mile-long shafts of morning sunlight splash against whole colonies of cliff swallows perched inside delicate clay-pot nests. Below that, peppering the walls, lie unreachable dark caves, offering refuge to weird rock-climbing cactuses. Farther down still lie the ancient fossils of clams and oysters and shellfish left by an ancestral sea. And down, down, down--farther down still--lie layers of petrified ash and flowing lava from prehistoric volcanoes.

Then, ground zero: the shimmering, shallow Rio Grande itself, where hand-size mud turtles, little changed in 50 million years, sun themselves on water-polished boulders. Narrow sandy beaches are flush with river cane and seepwillows and the tracks of ringtail cats.

Santa Elena Canyon imprisons you in its beauty. It dominates you from the moment you enter. There's no peeking over the tops of those walls at anything else the world has to offer. And climbing out is impossible. For the next 10 miles you get this world and no other, your raft a leaf floating through a universe scaled to giants.

A mile into the canyon, we begin to hear the rushing water of the Rock Slide rapids ahead. Jim, in the lead raft, starts explaining to the Georgians and Wisconsonians how to prepare for this tricky passage. In our raft, Jason turns to me. "We're going to get stuck up there in those rapids," says this 25-year-old with the red beard and ponytail. "There are passages between boulders that are narrower than the raft itself. But the raft will give a little if we push and pull and bounce around. We've just gotta work together on this."

The roar of crashing water gets louder and the river is strewn with massive boulders ahead. It's time. We secure our valuables inside steel 20-caliber ammunition cans and zip our life preservers like flak jackets. Jason has a knife strapped to his preserver so he can cut himself free if the raft flips and he's trapped underneath. I check and recheck my preserver. My mouth is dry.

Sure enough, at the very first rock "slot," where the river gets pinched between boulders just six feet apart, Jim's raft gets jammed. To help, Jason decides to row up from behind and give him a bump. But before the rafts touch, the air explodes.

A gunshot goes off right behind us.

It shatters our ears--bang!!!--creating an intensity of fright and confusion hard to describe. Jason and I instantly turn around to see a splash in the water just 10 feet behind us, physical evidence the attack is real. The splash is at the exact spot where our raft had been just five seconds earlier before Jason decided to speed up to bump Jim. My God. How can this be? We were nearly hit by the bullet. Who could be shooting at us? Why?

Reflexively we crouch low in our raft and Jason begins scanning the canyon rims for a sniper while I yell at Jim: "Go! Go! Go!" He's still maddeningly stuck in our only escape route from the line of fire. Whoever's shooting at us knew we'd be bottled up here; it's a perfect ambush site. We're sitting ducks, immobile at the bottom of a canyon.

Jim and the others struggle mightily against the rocks, oars in the boat, using their arms to push and pull against the boulders. Then Jason, still scouring the canyon rims, suddenly yells out: "Rocks!!! Rocks!!! Rocks are falling! Watch out!"

We hit the decks. From high on the canyon rim, two rocks the size of basketballs are hurtling directly toward us. Within seconds, two more gunshot sounds fracture the canyon air. Simultaneously, just 30 feet behind us, two more splashes send water geysering into the air.

It's a rock slide.

It's coming from the Mexican side.

There is no sniper.

Only later do we understand that the explosions we hear are coming not from the rocks violently slapping against the water surface. The sound is coming from the rocks barreling over 1,000 feet down the canyon cliff, entering the Rio Grande, traveling three feet underwater, then smashing against the rocky river bottom with such explosive force and speed that the column of air behind each rock doesn't have time to close before the deafening sound waves escape upward. Hence, gun blasts.

Never have I been so scared. It takes an eternity for Jim's raft to squeeze through the rock slot, all of us cringing through each crawling second, completely certain we'll be crushed at any moment by a shower of more rocks. To me it seems certain that the three ear-piercing blasts so far will, by themselves, trigger more avalanches on both sides.

Finally Jim gets free, then Jason and I pull through the slot with great effort. Then the two guides row like men possessed while the rest of us scour the canyon rims for more falling rocks. We see nothing, but we're far from out of danger. The rafts get stuck again in another boulder pass. Again we struggle free. Then we race down two sets of brisk rapids.

Finally, hearts still sprinting, we realize we're out of trouble. No additional rocks have fallen and we're well beyond the initial crisis spot. As the adrenaline rush subsides, it begins to dawn on us what a vastly unlikely thing has happened. No one will believe us. As if it weren't strange enough that rocks fell at all, almost hitting us out of the blue, it also happens that only three came down. Then no more. Nothing.

It was as if the canyon waited for the perfect moment, took its best shot, missed, then let us pass.

Later, a guide knowledgeable of the terrain up there on the Mexican side flatly ruled out the possibility of human involvement. People don't go up there, he said. Ever. Period. It's far too desolate.

So what caused the rocks to fall? The hoof of a wild horse? The echo of our own voices? The removal by wind of that last grain of sand in a 1,000-year erosion process that finally loosened those cliff-side bruisers? And why at the exact moment we were passing?

Jim and Jason, with all their years on rivers, had never seen a single rock fall. Forget almost being hit by rocks. They'd never even been able to say, "Hey, look. Did you see that rock splash in the river way up there in the distance?" Not once.

Assuming it was not related to tampering with the Tree of Death or some other riverine spiritual infraction, the only answer seems to be another question, the sort of question this vast and strange piece of terrain raises with its very existence. What are the chances that wind can create a sculpture resembling an Indian face? Or that the Marfa Lights happen? Or that a planet would even have deserts? Or that planets would circle stars, and stars gather themselves in galaxies? What are the chances?

Eventually, our necks sore from craning to watch the tops of the walls, we accept that we will not be "attacked" again and the river again becomes enjoyable. We have lunch, do some climbing and continue in our rafts. Then, with little notice, Santa Elena Canyon spits us back out onto the sprawling desert floor. The tall walls simply end, the vault disappears and the world has a horizon again. It's like waking up abruptly from the deepest possible dream.

I feel blue and confused as we drift away from the gorge, floating toward the takeout point. Only 10 feet separated me from death back there. Where's the uplifting insight into the preciousness of life? The meaning of the experience? Instead my thoughts are vague and uncertain, like the buckling shimmer of a heat mirage. The desert is fighting my memory, my need for a meaningful conclusion, holding on to its hushed doings to the very last.

I step off the raft and onto the desert surface. And, to my surprise, I keep right on floating.

Mike Tidwell last wrote for the Travel section about hitchhiking through the Louisiana bayou country aboard Cajun shrimp boats.

DETAILS: Big Bend National Park

WHEN TO GO: Big Bend National Park is one of America's most remote, scenic and pristine natural areas. Yet, ironically, if suffers from severe air pollution during the warmer months of the year when wind patterns bring in tons of sulfur dioxide and other pollutants from the Houston-Galveston area and from sources as far as Mexico City, 700 miles away. In the summer, the air over the park's unspoiled canyons and mesas can be as bad as anything in Manhattan during a severe temperature inversion. It's amazing anyone still doubts global warming is happening when even the atmosphere over our most remote sanctuaries is soiled for part of the year. Advice: Wherever you are, cut down on pollution by driving your car less and consuming less junk--and visit Big Bend in the autumn or winter for best visibility.

GETTING THERE: Continental and American are among the airlines offering flights out of the D.C. area to the Midland-Odessa Airport; round-trip fares start at about $324, with restrictions. From there it's a four-hour drive south to Big Bend. Or you can fly to El Paso, 4 1/2 hours from the park.

WHERE TO STAY: Abundant campsites exist throughout the park, connected by scenic trails. The park service also operates the charming Chisos Mountains Lodge (915-477-2291) with a view that can't be beat. If you're rafting the Rio Grande, stay at the Lone Star Ranch B&B just outside of Lajitas, Tex. (915-424-3403).

RAFTING: This is the best way to see the park's full array of gifts. Three area outfitters can arrange one- to seven-day trips. I recommend Big Bend River Tours at 1-800-545-4240.

INFORMATION: Big Bend National Park, 915-477-2251, www.nps.gov/bibe.

--Mike Tidwell