AS HE DOES EVERY WORKING DAY, Antonio Villarreal climbs into his rig, a .40-caliber handgun loaded and holstered on his hip. His 12-gauge shotgun and .223 assault rifle are upright in their racks. He checks the rearview mirror with weary cop's eyes and sets out on his beat.

Villarreal is a supervising ranger with the California Department of Parks and Recreation, responsible for the 3,600-acre Oceano Dunes State Vehicular Recreation Area, a revered pilgrimage site for what Dirt Sports magazine calls the "Off-Road Nation."

Villarreal's route takes him just above the high-tide line on this central California beach, past a scene out of Mad Max. Hundreds of battered dirt bikes, steel-pipe "rails"—little but frames, engines, and tires—painted in metal-flake crimson and chartreuse, four-wheel-drive pickups sporting skull and flame motifs, squat "quad" all-terrain vehicles, and others that look like mutant golf carts swarm the surrounding sands, spinning doughnuts near appreciative and pneumatic young women in Day-Glo bikinis, careening wildly during competitive sprints along straightaways, and leaping from the dune summits.

Two youths, skin covered with tattoos and acne, flag Villarreal down. Their four-wheel-drive truck has foundered in the sand and is threatened by the incoming tide. Their efforts to free the vehicle have only sunk it deeper, past the hubs.

"Dude, did you come to give us a tow?" asks one. Though no open containers of alcohol are evident, his eyes are glazed, and a fetor of fermented malt surrounds him like a nimbus.

The ranger regards the kid with flat eyes. "No," he says and hands him a card from a local towing company.

"We can't start offering tows," Villarreal says as he drives away. "If we did, that's all we'd be doing. Getting stuck is business as usual out here. Every year people lose their rigs to the ocean."

Along the beach, disappearing into the mist, is what looks like a vast, postapocalyptic settlement of campers and behemoth recreational vehicles, most towing trailers loaded with everything from gigantic propane barbecues to wading pools.

"On a typical summer weekend, we'll average 20,000 to 30,000 people on the beach," says Villarreal. "On a holiday weekend, we'll hit 60,000 or 70,000."

Birds wheel in the sky: gulls, brown pelicans, cormorants, and occasional flights of sanderlings and willets. Some appear to vocalize, but you can't hear them over the roar of red-lining engines, loud enough to set tooth fillings pinging.

"It's like any other small city," says Villarreal. "We average between one and three arrests daily. There are DUIs, assaults, rapes." He pauses. "And a lot of accidents."

Prodded, Villarreal essays a few terse recollections of crushed limbs, geysers of bright red blood from severed brachial arteries, and bawling children. People who died quietly or loudly. Incidents—many incidents—ending in paraplegia or quadriplegia.

"Over the years, I've become kind of desensitized to it, but it affects the younger officers very deeply," he says with a sigh. "Some require counseling."

THE VEHICULAR CHAOS AT OCEANO DUNES is replicated in deserts and wetlands and on hillsides and beaches throughout the United States—often completely unregulated. Off-road recreation in this country has grown enormously in the past three decades, jumping from 5 million users in 1972 to 51 million in 2004. For off-road vehicle (ORV) buffs, the risk to life, limb, and axle is just part of the testosteronic appeal. But public lands, particularly in the West, are paying the price.

In 2003, then–U.S. Forest Service Chief Dale Bosworth named "unmanaged recreation" one of the four biggest threats facing national forests. Former Bureau of Land Management director Jim Baca, now a spokesperson for a group called Rangers for Responsible Recreation, ranks the problem even higher: "Cumulatively," he says, "ORVs are doing more damage than any other single source."

Irresponsible off-roading does more than just rut hillsides and silt creeks. In one of the activity's more extreme permutations, known as "mud bogging," off-roaders drive into wetlands, crank their steering wheels, and hit the gas, with victory going to whoever excavates the deepest pits. Also gaining popularity is rock climbing—by machine. "We're starting to see people bringing bizarre, custom-made vehicles into the alpine areas of the forest," says Frank Mosbacher, a spokesperson for Eldorado National Forest in the Sierra. "They're using them to climb rock formations."

Today's ORVs are the bastard offspring of cheap, post-WWII surplus jeeps and stripped-down, souped-up Volkswagen dune buggies, born in the peculiarly Southern Californian car culture of the 1960s and '70s. Freeing motorists from the tyranny of the tarmac, off-roading made nearly any natural landscape a playground for mechanized humans. ORVs are merely the latest manifestation of a conflict that has bedeviled public-land policy since "auto camping" first came into vogue in the late 1920s.

Today the stakes are immeasurably higher, thanks to the confluence of population growth, advanced technology, and consumer affluence. While backpacking numbers are going down, the refinement of off-road engineering has yielded a wide array of vehicles at a multitude of price points, making off-roading, for many, the default way to interact with nature.

For some, the sport's implicit ethos is a willingness to run wild over not only nature but also the social contract. Last Easter weekend, for example, a bacchanal of a thousand ORV aficionados at the Little Sahara Recreation Area in Utah degenerated into a near riot. The 50 law enforcement officers who were summoned to the site clashed with inebriated riders for two nights running. The riders blocked roads and fondled women or forced them to bare their breasts before letting them pass. When the cops moved in, the riders pelted them with bottles, rocks, cans, and batteries. Thirty-seven people required medical treatment; 300 were arrested or cited.

Ron Kearns, a retired wildlife biologist and law enforcement officer who spent two decades working on the 665,400-acre Kofa National Wildlife Refuge in southwestern Arizona, says ORV use has spiked across the Southwest, with damage to desert lands increasing proportionally with the number of riders.

"There are 300 miles of sanctioned ORV roads at Kofa, but people persist in driving off-route," Kearns says. "For hundreds of feet on each side of the refuge roads, the desert has been rutted and gouged by ORVs." The passage of vehicles destroys the desert's fragile cryptobiotic crust—the thin, brittle veneer that can take centuries to form. Fracturing this "desert pavement" leads to rapid erosion and can destroy the algae, fungi, and other microorganisms that inhabit arid-land soils.

"I still live near Kofa, and I often see lines of 15 to 20 riders coming through," Kearns says. "You see more and more ORVs with the 'snowbirds'—retirees who come here each winter. Not too long ago, they were happy to hike. Now they're riding. Maybe they think it's their last chance to have some thrills."

The Forest Service is in the process of designating ORV routes in many national forests and proscribing ORV use elsewhere. But it is unclear how the understaffed agency intends to keep riders in sanctioned areas. Even in the Eldorado, located within an hour of 2.2 million people and one of the most visited national forests, only one law enforcement officer is on patrol. ("We plan to expand that to four," says Mosbacher.) When wayward riders are caught, the penalty is minimal: "The fines are too low," says Kearns, "about $75."

For their part, ORV enthusiasts say the sport has been smeared by the antics of rogues. "Sure, there are a few knuckleheads—maybe more than a few," says Bill Dart, land-use director for the Off-Road Business Association. "But the picture painted [by environmentalists] isn't accurate. Most ORV users are law-abiding and considerate." Dart complains that environmentalists "are adamantly opposed to motorized recreation under any and all circumstances. There's no room for talk."

Jeff Ruch, executive director of Public Employees for Environmental Responsibility, finds this disingenuous. "Their approach," he says, "is basically 'F— you; why don't you talk to us?'" Discussion with ORV enthusiasts, he says, must begin with their acknowledgment and respect for sanctioned off-road routes. "Otherwise, off-road use precludes all other wildland uses. It's hard to birdwatch if there's a mufflerless motor screaming nearby."

Most environmental groups, including the Sierra Club, do not argue for the elimination of off-road vehicles, says Karl Forsgaard, chair of the Club's Wild Planet Strategy Team. But he does encourage hikers, skiers, and others to assert themselves. "I'll tell you what doesn't work," he says. "When you're in a meeting with one ranger and 90 motorheads. The general public is behind us, but off-roaders are able to mobilize their forces very effectively."

Baca, of Rangers for Responsible Recreation, has an idea of how to rally the nonmotorized millions. "I have the perfect TV ad," he says. "A family is in their backyard, admiring the new lawn they've just put down. Then a guy hops the fence and tears it up with his ATV. In essence, that's what's happening on our public lands."

Glen Martin was, until recently, an environmental reporter for the San Francisco Chronicle.

To join other Sierra Club activists working on ORV issues, go to sierraclub.org/recreationissues.

Thrillcraft The photographs in this article also appear in Thrillcraft: The Environmental Consequences of Motorized Recreation, published by the Foundation for Deep Ecology. Edited by George Wuerthner, the book includes essays by Rick Bass, James Howard Kunstler, Howie Wolke, and others.