Before the phrase "sprockethead" entered the local vocabulary, before cycle store owner Bill Groff started selling 10,000 cyclist's water bottles a year, and long before anyone dreamed that this town would become the mountain biking capital of North America, Moab was already an improbable place.

It sat here in the southeastern corner of Utah, surrounded by dinosaur tracks, Anasazi petroglyphs and billions of tons of sandstone, baked, warped, exploded and decayed into a desert-scape of thousand-foot fins, blood-red mesas, unengineered arches, gritty winds, gnarled junipers and a deep, damp, crooked gash in the earth known as the Colorado River. Beneath its crusty skin, the place was stranger still: Uranium lay here in enormous quantities. Forty years ago, when the leaders of the American nuclear industry went looking for the power to do unearthly things, they ended up digging under these rocks.But Utah uranium isn't in demand the way it once was, and for the last five years Moab's future has been rolling in on knobby rubber tires. Moab is where mountain bikers migrate to careen down the inclines of the 10-mile Slickrock Trail, to glimpse the snow-capped La Sal Mountains, to explore hundreds of miles of rock-strewn old mining trails, to torture $2,000 bicycles. To the north sprawls Arches National Park. To the southwest, Canyonlands National Park.

"I love this!" huffs Dina Kilgo, 25, a vacationing teacher from Fairfield, Calif., pumping her pedals toward the nearby summit of Poison Spider Mesa on a hot, dry afternoon. To get here, almost 5,000 feet above sea level and 1,000 feet above the trail head, the helmeted Kilgo has negotiated seven miles of path that Todd Campbell, guidebook author and bike-path sage of Moab, has described as "gonzo abusive." She is ecstatic.

There are thousands more like her. Bicycle traffic on Slickrock, the area's most popular trail, will probably pass 100,000 riders this year.

Most of the area's trails were cut decades ago by off-road vehicles on land now controlled by the U.S. Bureau of Land Management (BLM) or the National Forest Service. Jeeps and motorcycles can legally share most of these trails - a "Jeep safari" early this month drew more than 1,300 four-wheelers to local trails - but on most days, bicycles now outnumber motorized vehicles 100 to 1.

In town, bicycle culture burgeons. In a community with 7,000 residents and a Main Street fewer than 10 blocks long, three bike shops thrive, each with a steaming espresso machine out front. The taxicabs feature roof racks - the better to haul you and your bike to a trail head outside town. Even crime statistics - puny by any urbanite's standard - reflect the rise of the bicycle. Police chief Alan West counted just 14 car thefts in town last year, but 41 stolen bikes.

Retrieving those bikes, by the way, is not always difficult.

"You see one of our local low-lifes riding a $5,000 bike," says West, "and you go, `Aha!' "

Blue sky, red rocks. A brilliant, chilly April morning. Six of us are jouncing out of town in a four-wheel-drive Ford toward the Gemini Bridges trail. Our bikes (most of them rented for $20-$25 per day) are clipped to the roof, and trail guide Maggie Wilson of Kaibab Bike Tours is slurping highly potent coffee and briefing us on the 14-mile ride ahead.

"If your body and your mind say `Whoa!' listen to that," Wilson is saying. "Otherwise ... it's Biff City."

I'm no expert on Utah geography, but I know I don't want to go there. Wilson goes on with advice on braking (use the rear brakes more than the front) and etiquette. We should stick to the well-trod trail, thereby avoiding the cryptobiotic crust that coats much of these hills and directly or indirectly sustains virtually all plant and animal life in the desert. (The best biking and recreation maps of the area are published by Latitude 40 in two parts, Moab East and Moab West, and are widely available in town, usually for about $8 each.) Like many of Moab's year-round cycle people, Wilson has a background in environmentalism - biology bachelor's degree, University of California, Santa Cruz - and choice words for riders who stray from trails.

I strap on my helmet, poke toes into the bike stirrups, aim downhill, and start operating my jackhammer ... that is, riding my bike, which, despite the fancy shock absorbers up front, feels like a piece of heavy machinery striking concrete rubble. The trick of downhill mountain-biking, experts say, is to absorb those blows with one's arms, keep one's center of gravity low and toward the rear wheel, and balance the bike in a sort of controlled forward hurdle. Heading uphill, the key is keeping your legs churning, conserving momentum in gear-shifting, and guiding your tires toward ground that allows maximum traction.

And if bumpy biking doesn't agree with you, there are rides on smoother trails. Or you can ditch the bike and go for a hike, or sign up with one of the outfitters running rafts on the Colorado River.

The day after the Gemini Bridges ride, I explore Hurrah Pass, another 14-mile route for beginners. The trail flanks the Colorado River and peels off alongside Kane Creek, climbing, winding and swooping through shade and sun. "Physically easy/technically easy," according to Campbell's guide. Fine by me. Here a rider can round a corner, lean into a hard climb, and find his world reduced to the path below, the sky above, the immense canyon walls on either side, and that merciful arrangement of chain and sprocket that cyclists call "granny gear." Time slows down, and all you hear is the desert rustling and the wind racing.

Given the time and energy, I'd ride the 16.5-mile Monitor and Merrimac trail next, then maybe the 25.4-mile Back of Behind, or the 25.2-mile Courthouse Loop. A rider could stay weeks and not run out of possibilities.

But it only makes sense to venture out on these trails when temperatures are reasonable. In July and August, highs reach between 95 and 100 degrees. In winter, temperatures dip below freezing. April and October bring the best weather for biking (though April also brings throngs of students on spring break and ski bums just released from seasonal jobs at Colorado's resorts). Every year the season semi-officially ends with a Fat Tire Festival and Halloween party.