WILLIAMS - Somewhere out there, there's a modern Western explorer who decided he had something so important to say that it had to be slathered in silver paint on a remote rock wall full of ancient petroglyphs in the national forest.

The mysterious etchings depicting people, animals and a blazing sun are in a box canyon known as Keyhole Sink in the Kaibab National Forest east of Williams, a mountain town off Interstate 40 that has welcomed sojourners since its namesake, fur trapper "Old Bill" Williams, explored the locale in the early to mid-1800s.

The pristine rock art in Keyhole Sink was a silent reminder of the ancient culture that long flourished in northern Arizona, and it stood unaltered for at least 1,000 years. That all changed in August, when someone painted "ACE" on top of the petroglyphs in sloppy, dripping letters. Under the defacement is an indistinguishable glop of paint that could be more lettering.

Arizona's defaced artifacts

Kaibab officials aren't sure exactly what it says, nor what it means, other than a potentially expensive restoration job that might not work. Investigators are trying to find the culprits but have no suspects.

"It's beyond words," Kaibab archaeologist Neil Weintraub said of the damage. "It feels like an attack on this site. What has it done except give people pleasure for years?"

The damage at Keyhole Sink is a fresh reminder of the ongoing assault on ancient archaeological sites in Arizona and across the Southwest - graffiti, looting of artifacts, littering and garbage-dumping. Sites are defaced with paint, bullet marks, paintball stains and messages scratched into rocks. Professional thieves remove pottery, hack out chunks of ancient art-covered rock and dislodge anything they can carry away.

The sites are vulnerable because they're not behind locked doors. They are operated on the assumption that visitors will behave, since monitoring is intermittent at many of these locations. There aren't enough people, either paid or volunteer, to check them frequently. There are simply too many sites. Often, they're hard to reach.

"We can't monitor them all, and neither can the land managers," said Nicole Armstrong-Best, interim coordinator for Arizona's Site Stewards program. The program oversees a group of volunteers who monitor local, state and federal sites all over the state.

There are about 800 volunteer stewards who monitor the 3,000 most significant or most affected sites the program tracks. Armstrong-Best said there are thousands of other sites - both known and undiscovered - not being watched.

More than 130 vandalism reports have been filed by the stewards since October 2009, when a computerized reporting system was put in place. Reported incidents include petroglyph thefts, paint damage, graffiti and dumping of debris. In a few cases, even shrines and cairns have been built on the sites, along with other alterations.

Looters and vandals can be prosecuted under the Archaeological Resources Protection Act of 1979. But experts say the cases can be difficult to prosecute unless there are witnesses. Still, there have been enforcement actions in Arizona and neighboring states recently.

Perhaps the best known is a federal sting that targeted looters in the Four Corners area of Arizona, Colorado, New Mexico and Utah. More than 25 people were arrested in the case.

In other examples of prosecution, a Bullhead City man was hit with several citations after a paintball fight damaged petroglyphs in the Lake Mead National Recreation Area, and the federal Bureau of Land Management is investigating a case near Tucson in which people were spotted spray-painting a series of rock-art panels.

Archaeology buffs like Robert Schroeder of Phoenix wonder if it's a good idea to have the sites listed so publicly.

"I don't see any easy solution," said Schroeder, who photographs petroglyphs, including some that have been damaged. "You want Americans to have access to the country's cultural resources, but you want to keep sensitive sites off the radar, so to speak."

Mike Johnson, deputy preservation officer for the BLM's Arizona office in Phoenix, said urban growth in the West means more people looking to crowd into diminishing open space, putting more pressure on archaeological sites. At the same time, he said technology like GPS helps people find sites, and Internet marketplaces permit thieves to easily market what they've stolen.

Johnson said the BLM is working to increase steward visits and patrols by uniformed officers at sensitive sites, and to increased cooperation with Native American tribes, for whom these sites are sacred reminders of their ancestors.

Experts say that the impulse to restrict access is giving way to the idea that educating visitors about the value of the sites will make them more apt to notice and report criminal shenanigans.

"You give people who are professional looters more reason for concern, more eyes and ears out there," said Andy Laurenzi of Tucson's Center for Desert Archaeology.

Kaibab officials were proactive in trying to protect Keyhole Sink. Two forest roads leading to the small canyon were closed, and now the rights of way are carpeted with forest growth. Without easy access, garbage and litter almost disappeared.

To keep the site accessible, forest managers created a ��-mile walking trail to Keyhole Sink. They figured that anyone willing to make the effort to get there on foot would value what they were seeing.

Kaibab officials erected signs warning against vandalism and explaining the significance of the site. Now, Weintraub said, the agency may have to consider installing cameras and motion detectors to protect the site, though that runs counter to his notion that the place is a touchstone to the past.

Until the paint is removed, he said, people who come there from around the world will be disappointed.

"We've lost the value of people being able to come there, see the stuff, sometimes sit there alone and imagine how it was for the ancient people who lived there," he said.

Margaret Hangan, Kaibab National Forest's heritage-program manager, said she was unsure if anyone would be caught for the vandalism. Kaibab officials are still trying to figure out the best way to remove the paint without leaving more damage. Officials hope publicity helps educate people and generates some tips on who did it.

"It hurts us emotionally, because this is just such a special place," Hangan said recently, standing near a pool at the base of a cliff where a waterfall cascades during the snowmelt season. "It's really hard to see that not everybody feels the same way we do about it."