very weekend, from the suburbs of Columbus and Cleveland and Cincinnati, hundreds of big-bed pickup trucks and trailer-toting SUVs pour onto the narrow dirt roads of Wayne National Forest, each of them hauling two or three ATVs or a half dozen dirt bikes made by Suzuki, Honda, Polaris, Yamaha, and Kawasaki. It's virtually an all-male crowd, ages 13 to 55, with only a handful of girlfriends and wives who faithfully tend the charcoal fires and microwave ovens, awaiting return of their men from a day of hard riding.

They come here to the southeastern corner of Ohio because, in an otherwise flat state, the gnarled Appalachian landscape provides the kinds of challenges to man and machine that ORV riders relish -- steep slopes to climb, streams to ford, hillocks to leap, mud to splatter, hairpin turns to maneuver, and 40-yard straightaways where a Yamaha 250 can crank up to 60 mph.

It isn't a sport without thrills, I discover, riding as a passenger on 14-year-old Caleb Winfield's dirt bike and, later, driving 41-year-oldDan Avery's ATV at Dorr Run. (Since both these riders let me ride their machines illegally, I've changed their last names.) But, except on the level, I'm terrified riding these open, unprotected machines. Cresting a ridge or speeding downhill, I have the same paralyzing fear I experience in a roller coaster when the earth disappears and I'm about to hurtle into space. And my fear is not unjustified; every one of the two dozen riders I talk with has broken an arm, leg, rib, or collarbone by smashing into a tree, a boulder, or another rider or by taking a self-sacrificing spill to avoid flattening an unexpected deer hunter.

The riders make no pretense that their sport has anything to do with nature or the outdoors, except as a series of topographic obstacles: "The scenery is irrelevant," says Avery. "You're moving fast. Half the time you're in dust so thick you can't see 10 feet. All you can do is focus on your front wheels."

oday, from the Florida marshlands to the Rocky Mountains to the Alaska tundra, millions of off-road-vehicle riders regularly recreate on public land. To hear the riders tell it, full-tilt ORV sports represent the ultimate in personal freedom. But for the nation's nonmotorized outdoorsmen and for state and federal land managers, the popularity of off-road recreational vehicles has created problems never before seen in the American outdoors.

Technically, any engine-powered machine used off-road -- whether it's a motorboat or the family sedan -- qualifies as an ORV. But the list of the most problematic factory-made vehicles, usually built for only one rider, includes dirt bikes and all-terrain vehicles, along with their wintery and watery counterparts, snowmobiles and personal watercraft, often called by the brand name Jet Skis. (Sport-utility vehicles are surely capable of modest off-road travel, but, as everyone knows, their owners seldom use them farther off-road than the nearest shopping mall.)

The defining feature of all these machines, their critics say, is that they are thrill vehicles (or "crotch rockets"), used not to transport passengers innocuously from point A to point B in the outdoors -- as, for example, a tour guide's van might be -- but for the rigors and excitement of the ride itself. "ORVs are built, advertised, and used for speed and competition," says Bethanie Walder, executive director of Wildlands CPR, a Montana-based environmental group. "We don't say they should be outlawed, but if you want to ride a vehicle on public land, you should be on a road. If you want to get off the road, you need to get off your vehicle."

Here in Wayne National Forest, the U.S. Forest Service in the mid-1990s built off-road-vehicle riders a 118-mile trail system of their own, leading through roughly 26,000 acres of public land. In digging new, wide trails for the vehicles, the Forest Service attempted to include all the twists, turns, and climbs ORV riders appreciate. It built new trailhead parking lots for their trucks, motor homes, and SUVs.

At the same time, the agency took steps to limit the land damage caused by the heavy, tire-spinning vehicles and to keep them away from the forest's traditional hikers, hunters, campers, and horseback riders. It built dozens of rugged wood and steel ORV bridges across creeks, added concrete reinforcing rods and thousands of tons of gravel to "harden" its dirt trails, and erected metal gates and wooden fences to block ORVs from entering the 210,000 acres of Wayne National Forest where vehicular travel is prohibited. The Forest Service also clearly marked its ORV routes with hundreds of formal wooden signs and with thousands of orange metal patches nailed to tree trunks, and it issued a legal order requiring that the machines stick strictly to those trails.

But it's clear at a glance that the agency's attempts to control the vehicles are regularly and illegally thwarted. At a Forest Service bridge across Purdum Creek, I watch while 14 out of 22 ORV riders reject the bridge and instead plunge through the streambed immediately alongside, because it's a chance to spray water and mud. At Dorr Run and the South Woods, I observe a half dozen places where Forest Service signs -- "Closed. Not a Designated Trail" -- have been broken off their posts or riddled with bullets, and I see fences and gates that have been smashed down by riders to gain access to areas of the forest that are off-limits to them.

"They get bored with the kind of trail experience we're offering and they wander off," says Mike Baines, former Wayne District Ranger who now works in the Monongahela National Forest in West Virginia. "But when one ATV rider wanders off an established trail the next rider and the next will see his ruts and follow him; and then you've got a brand-new trail. If we give a $50 summons to the fifth or sixth illegal rider that day, he'll say, 'Hey, why? I'm just following an existing trail!'"

Nationwide, land-managing agencies ranging from the Forest Service to the U.S. Bureau of Land Management, the National Park Service, and the U.S. Fish and Wildlife Service, which together administer more than 625 million acres of public land, openly acknowledge that they do not have the money or manpower to protect the American outdoors and the nonmotorized public from year-round incursions by today's potent off-road vehicles. If they completely close their borders to ORVs, land managers say, the riders will enter anyway and churn up the landscape at will; there's no fence or gate or law that will keep them out.

But if they accommodate the machines by specially building them 50-inch-wide and hardened vehicular trails, ORV riders simply use their new trails as jumping-off points to illegally penetrate even farther into an area's fragile backcountry. Every year these machines push just a bit deeper into the wilds -- up to Anderson Pass in Denali National Park, into Georgia's Rich Mountain Wilderness and the northwest corner of the Arctic National Wildlife Refuge. "The poorly managed use of off-road vehicles in Alaska results in more physical deterioration of wild areas than all other uses combined, aside from clearcutting," says Ray Bane, a former superintendent of Katmai National Park in Alaska. The illegal riders are almost never caught, not just because their machines are speedy but also because, on public land, an individual ranger or forester is typically assigned to patrol an impossibly large area, anywhere from 125,000 to 2.5 million acres.

In an effort to induce ORV riders to confine their most land-damaging activities to a relatively small area, dozens of state and federal land managers from Georgia to Illinois to Oregon to Alaska have, off the record, adopted a policy of designating "sacrifice areas" -- riders call them "play areas" -- so enthusiasts can charge up and down slopes, through streams, across desert and marshes to their heart's content, utterly pulverizing large areas of public land. The hope is that this policy will displace the behavior from wild and more scenic areas nearby.