The Big Cypress National Preserve in Florida is crisscrossed with so many illegal swamp-buggy ruts - more than 23,000 miles of them - that park officials in August began limiting off-road vehicles to 400 miles of trails in order to protect the Florida panther and the preserve.
In Yosemite National Park, off-road vehicles are involved in "numerous" violations, according to a staffer's memo.
And each day of a long summer weekend like Labor Day, as many as 2,200 motorized vehicles hit the beaches of Cape Hatteras National Seashore in North Carolina. "Not one of those is there legally," says Don Barger, southeastern regional director of the National Parks Conservation Association, a watchdog group.
In all, unauthorized off-road vehicles are buzzing through nearly one-third of America's national parks, according to a recently released internal National Park Service (NPS) survey. In one-fifth of the parks, they have damaged natural environments that by law must be preserved for future generations.
The survey highlights a conundrum for the park service. It has gone to bat for broader access for motorized off-road vehicles - including snowmobiles in Yellowstone National Park. At least 29 of its parks have allowed illegal off-road vehicles. On the other hand, in some cases, the parks are prohibited from doing so by their authorizing legislation; in other cases, they haven't conducted required environmental impact evaluations of the vehicles, according to a legal analysis by Bluewater Network, an environmental group in San Francisco.
In March, the NPS convened a workshop of rangers and other park officials to develop a "national strategy" for dealing with the problem, according to a senior park official.
NPS officials downplay the challenge.
"Some of the illegal use is very infrequent. Some of it is crossing boundaries that may not be well marked," says Jerry Case, the service's regulations program manager. "We're certainly working on it. [But] there are probably just a handful [of parks] that have significant illegal use causing significant environmental damage."
The Cape Hatteras seashore is one facility that until recently had not conducted the required legal reviews to allow off-road vehicles, though it is doing so now.
"We certainly recognize that some [facilities] need to get back in line with the executive order and regulations, but for the most part I think we're doing all right," Mr. Case says.
Other branches of the Department of Interior, however, say illegal motorized vehicles are a serious national problem. The chief of the National Forest Service, for instance, has called it one of his top four management problems.
An Interior Department spokesman told Congress in July that 43 of the nation's federal "park units" allow snowmobiles, 25 allow off-road vehicles, and nine allow personal watercraft.
But he made no mention of any problem with the illegal vehicles highlighted by the internal survey.
Critics say the problem is acute and growing, even if not yet felt uniformly. "The pressure to increase these kinds of uses in national parks is constantly increasing," Mr. Barger says. "Our park lands are becoming more and more like isolated islands, with the demands for their use from everybody."
At least 113 of the nation's 388 park units report illegal off-road vehicles, the internal survey showed, and 85 of those report that the vehicles have caused environmental or other damage. Striking as those results are, they may understate the scope of the problem, says Sean Smith, public lands director for Bluewater Network. Only 273 of the 388 park units had responded to the survey. Parks like Yellowstone, Acadia, and Shenandoah National Parks had not responded.
There are signs of movement. Sections of Cape Hatteras were closed this summer to limit damage to nesting piping plover. The tiny shorebird's chicks are often trapped in the ruts left on the beach by off-road vehicles and run over.
But in the survey, many parks reported they lacked the staff or funding needed to chase down the illegal vehicles, says Bluewater's counsel Robert Rosenbaum of the law firm Arnold & Porter in Washington.
"Operators are rarely caught due to staffing limitations," wrote a park official in the survey from Arches National Park and Canyon lands National Park in Utah, cited in a June letter by Blue water to the Park Service. "There are a number of locations where off-road travel has destroyed microbiotic soil crust, destroyed desert vegetations, crushed animal burrows ... impacts that may last for years."
Although Yellowstone National Park was not part of the survey results, a now infamous example occurred in 2003 when two men on off-road vehicles damaged Lone Star Geyser by driving around its cone and in surrounding meadows, according to park reports. Just last year, other illegal drivers cornered a herd of bison in a stand of trees at the park.
"To allow that kind of thing just because a set of recreational users want to take snowmobiles and whatever else into Yellowstone is crazy," says Bill Wade, a former superintendent of Shenandoah National Park, now a spokesman for the Coalition of National Park Service Retirees, a watchdog group based in Tucson, Ariz.
"It's like me saying that because I like skeet-shooting, I should be able to do that in the Sistine Chapel," he says. "There are some places with such high degree of reverence [vehicles] just should not be allowed."