It's hard to take the Great Sand Dunes seriously. Nature has worked for eons on this silent sight gag of a joke, piling up acres of sand at the base of the Sangre de Cristo Mountains in southern Colorado. They are the tallest dunes in North America, almost 700 feet high, and they couldn't seem more out of place. You can see them for miles as you travel through the San Luis Valley. They take shape as a mirage, only instead of an oasis, you see a desert stacked right up against the mountains at the eastern edge of this high, flat basin.

The Great Sand Dunes National Monument was established in 1932. Every spring, when the high country is still deep in snow, crowds converge on the massive sand pile with picnic baskets, beach umbrellas and plenty of sun-screen to pretend it's summer. State college students drop in for a quick, bargain-beach tan.

On Memorial Day weekend as many as 35,000 visitors enter the monument. "That's when they almost love us too much," said Robert Reyes, National Park Service superintendent there. This is the peak of the May-June runoff, when melting snow from the Sangre de Cristos fills Medano Creek, and it flows past the southeastern edge of the dune field. The rest of the year, "the disappearing river" sinks into the sand above the visitors' center.

The stream seems almost magical with its periodic surging waves created by drifts called anti-dunes, which form and reform on its steep, sloping bed of deep sand. Children with buckets and shovels, or bare hands, construct endless variations of castles and dams while the water is still, then watch them wash away in the next pulsating wave.

To reach the dunes at this time of year, you must take off your shoes and ford the wide, shallow river, which may run two inches to two feet deep.

The dunes themselves are never crowded -- the main dune field covers 50 square miles -- and you can easily get disoriented. Distances deceive in this barren sandscape of rippling light and shadow. It takes 45 minutes to 1 1/2 hours to reach the top, depending on the heat of the day and the fluidity of the sand. Climbing is easiest after a rainstorm, when the sediment is packed. At other times, your feet sink ankle deep into the fine powder, and it's two steps forward and one back, spilling mini-avalanches all the way.

There are no trails. You are an explorer, a sheik, a space traveler, the first (or the last) person on earth. The view from the top is pure fantasy, an alien world of abstract contours changing with the light. The wind creates plumes from the crests while it blows grit into an unprotected camera. The elevation reaches 8,690 feet above sea level, so a jacket is sometimes needed. It's not a place to hang around in a summer thunderstorm, either.

Die-hard skiers carry their old "rock skis" to the ridges and plow a course down. Other adventurers descend by rolling, leaping and running, especially those foolish enough to expose bare feet to the surface heat, which can reach 140 degrees Fahrenheit. Shoes are a necessity.

Flora and fauna are sparse on the dunes, where the average rainfall is close to 10 inches annually. But the list of mammals in the vicinity would delight Beatrix Potter. Little brown bat, long-eared myotis, pika, least chipmunk, Apache pocket mouse, bushy-tailed wood rat and desert cottontail rabbit are some of the small inhabitants of the surrounding grasslands and forest, along with porcupines, raccoons, badgers, coyotes, pronghorns, mule deer, bobcats and an occasional mountain lion. Among the only critters comfortable on the dunes, though, are the kangaroo rat -- which never needs to drink water -- the giant sand treader camel cricket and the Great Sand Dunes tiger beetle, both of which live nowhere else in the world.

The force behind the dune-building is the wind, which sweeps across the 50-mile-wide, mountain-rimmed valley from the southwest, carrying debris all the way from the San Juan Range on the western edge. This plain may once have been an inland sea. It has long been a drainage basin for mountain streams, principally the Rio Grande and its tributaries, and it is covered with deposits from 15,000 years of erosion.

The prevailing wind propels this sand toward three low passes in the eastern mountain barrier (Medano, Mosca and Music passes). There, in the turbulence, the sand is dropped into natural pockets at the base of the Sangre de Cristos. Reverse winds through the same passes stabilize the dunes, holding them in place. These winds probably produce the sounds of the "singing sands" celebrated in local song and poetry, but seldom heard.

The motion of the dunes is imperceptible, but in their shifting the dunes have buried a coniferous forest, leaving in their sandy wake skeletal "ghost trees," as well as other skeletons and artifacts dating back to Folsom man.

The San Luis Valley seems to exist outside of time. Indian hunters and Spanish colonial explorers trekked across this harsh land. Anglo trailblazers and gold seekers crossed its 14,000-foot mountains. Hispanic settlers in San Luis, Colorado's oldest town, and others like it brought a language and tradition still linking their descendants to the past.

When the Sangre de Cristo peaks are awash in crimson at sunset, their name, "blood of Christ," evokes the mystery of this land. Folk tales about the dunes are not surprising -- stories of vanishing sheep herds, web-footed wild horses racing over the sands in the moonlight or betrayed Spanish lovers walking through the night. Journalists have added to the mythology with photographs of camels and, in the 1970s, a mock-up of a Mars lander (in National Geographic) on the site. Then there's the prankster who planted a fake drinking fountain high on the dunes to tease the tourists.

"People who leave the beaten path to come here are especially appreciative and interested," said a former superintendent, Jim Carrico. The dunes are a wilderness area, a geologic curiosity, a playground and a wondrous sight. Zebulon Montgomery Pike, exploring the territory in 1807, compared them to "a sea in a storm." As you drive away from the undulating hollows and peaks, they seem unreal again, an illusion painted in cream, gold, green, pink or lavender, depending on the light.

Jean Cramer is a free-lance writer living in Colorado.