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PHOENIX - In the dead of night, looters are destroying the history of America, desecrating sacred Indian ruins.

An estimated 80 percent of the nation's ancient archaeological sites have been plundered or robbed by shovel-toting looters. Though some of the pillaging is done by amateurs who don't know any better, more serious damage is wrought by professionals who dig deep, sometimes even using backhoes.

The motive is money. Indian artifacts are coveted worldwide by collectors willing to pay for trophy pieces of the past.

Looters are just the first link in a chain that includes collectors, galleries, trade shows and Internet sites such as eBay. But stopping the black-market business is virtually impossible because of a lack of manpower for enforcement and loopholes in the law that make it hard to convict the few who get caught.

The result is a scientific and spiritual loss.

''They're changing history,'' Vernelda Grant, a tribal archaeologist for the San Carlos Apaches, says as she stands amid 800-year-old ruins that have been transformed into a crater field. ''They're killing us. They're killing the existence of who we are.''

Garry Cantley, an archaeologist with the Bureau of Indian Affairs, does not share the mysticism; he lives for empirical discoveries, the piece-by-piece puzzle of history, the cultural window. But, like Grant, he is sickened by the damage.

''The problem is, they don't make these anymore,'' Cantley says, surveying the field of foxholes. ''The archaeological records are finite. And, once they're gone, history is gone.''

The San Carlos Reservation covers 1.8 million acres of high desert, pine forest, canyon lands and archaeological sites - a wilderness patrolled by 10 rangers who spend most of their time protecting game and fish.

In May, a report by the National Trust for Historic Preservation concluded that artifact hunters, off-roaders, urban sprawl and vandals are ''robbing the nation'' of cultural resources.

Warren Youngman, assistant BIA special agent in charge for Arizona, shrugs when asked how many looters are working tribal lands: ''There's a lot of wide-open spaces, and we don't have the manpower to cover it. We'll never know.''

Until this year, the BIA, with policing oversight for 561 recognized tribes nationwide, had just one investigator assigned exclusively to looting. The agent, John Fryar, retired this year and was not replaced.

''I just barely scratched the surface, frankly,'' says Fryar, now living in New Mexico. ''One person was definitely not enough.''

The lack of enforcement is true across a nation peppered with ancient settlements in national forests, federal parks, BLM lands, military bases and state turf. Just two investigators work Arizona trust lands covering 9 million acres. BLM officers cover more than a million acres each.

Meanwhile, it is sheer guesswork as to what percentage of ruins have been looted.

A 2002 report on federal lands in the remote Four Corners area put the figure at 32 percent. Archaeologists and enforcement officers generally estimate that eight of 10 Southwest sites have been robbed or damaged.

Using a GPS device, a professional digger can read the landscape and quickly map out a 1,000-year-old village that has eroded into the earth.

Some ruins resemble minefields, full of holes and dirt piles.

Cantley, the BIA archaeologist, says hard-core looters school themselves in archaeology and zealously defend their right to dig.

''These guys know archaeological sites as well as the experts,'' he says.