SUNDANCE (AP) - Driving in his pickup from Sundance on U.S. 14, Gene Gade points out the site's location, virtually invisible from a distance to the untrained eye.

"You don't get the feeling that there's a big hole over there," said the bearded, stocky man, pointing along the ridge that forms just along Interstate 90.

After a few more minutes of driving, Gade stops his truck and gets out, passing a couple of ragtag signs that show-and-tell the tale of the site.

Just beyond the signs, it is finally visible: a 200-foot-wide sinkhole, a piece of land that caved in when the roof of an underground cavern collapsed.

At first, it might not look like much.

"Some people are disappointed when they first see it," Gade said.

The odds are good that passers-by on I-90 won't even notice the Vore Buffalo Jump site. But it was here for 300 years that Indian tribes from across the western and central plains killed and slaughtered thousands of bison.

Evidence at the site shows that a variety of tribes - the Kiowa, Plains Apache, Absaroka, Shoshone, Hidatsa, Mandan and Arapahoe - may have used the buffalo jump. Gade said oral accounts written down in the 20th century show that the Northern Cheyenne used it as well.

Hidden in the hole are the bones of an estimated 20,000 buffalo, plus thousands of Indian artifacts, weapons and tools.

The irony of the site is that the subtlety that allowed it to become so successful in the first place by tricking thousands of buffalo to stampede over its lip to their deaths also keeps it from being noticed today.

Those who do come have only a few simple rules to follow: no digging or disturbance of the site in any way. No object, plant or animal life may be removed.

"Unauthorized digging here is like tearing the pages out of an ancient manuscript," Gade said.

Gade is president of the Vore Buffalo Jump Foundation. He is also a University of Wyoming extension educator based in Sundance.

As well as anyone, he visualizes what the site once was, and what it one day could be.

"This isn't just a pile of bones," Gade said before venturing to site's center. "This is history."

Tribes started using the site around 1500. Its last known use came in about 1800.

After pausing at the rim, Gade chooses to take the long route to the bottom, a dirt path that spirals into the very center of the pit. Looking from the inside out, somehow the whole thing seems much, much larger.

In actuality, it's up to 40 feet deep. The site was even deeper when it was in use, but it has filled up at least 20 feet in the last two centuries.

Still, it's not hard to discern that any living thing falling over its edge would be in serious trouble.

"If you were running off this at top speed and had hundreds of other 1,200-pound animals falling in behind you, it would have been an `owie,"' Gade said.

Up the west slope is another dirt path. At roughly 114 paces, it takes a few dozen or so fewer steps to travel than the long route from top to bottom, but it is much steeper and more difficult to walk.

It also has more historical importance.

"This is the actual Indian trail where they would've carried off the meat," Gade points out.

As Gade looks over the sinkhole before making his way up the old trail, cars whiz past barely tens of yards away on I-90, most of them likely unaware of his presence or that of site.

"They just keep on buzzing by, assuming nothing is here," Gade said. "This thing is literally right next to the interstate. In fact, they had to move the interstate to accommodate it."

In 1970, the Wyoming Department of Transportation was planning I-90 through the area. When initial tests showed a solid layer of bone, state archaeologist George Frison arrived at the scene.

When a couple more tests revealed some Indian bow and arrow points, Frison knew he was onto something. He helped get National Science Foundation grants for 1971 and 1972 to research the site, and the interstate had to be moved slightly.

UW archaeologist Charles Reher, archaeological director of the project, said it was evident early on that researchers had stumbled onto a unique find.

"There's nothing like it in terms of preservation and accessibility in the world," he said recently. "There's very few like it that would be similar in scope and size."

What makes the site so special is that as a sinkhole - as opposed to a cliff, like most other buffalo jumps - it allowed for the gradual, natural accumulation of sediments. The sediments in turn preserved the remains and artifacts from the bison hunts in stunning detail.

"It's a little Pompeii-ish in that sense," said Gade, referring to the famous Italian city whose residents were frozen into position by volcanic ash in an A.D. 79 eruption.

The sediments also gradually piled up in layers for each year and, like the rings of a tree, precisely document the age of the site's findings. By looking at the rings, archaeologists often can tell within one to three years when an object made its way into the hole.

The rings of sediment also tell what the climate was like. Thick rings indicate wet periods, and thin rings indicate drought.

"It's like a 300-year weather station," Gade said.

Most major buffalo hunts occurred in the fall when tribes were trying to stock up on food and supplies for the winter. And by examining the buffalo bones, scientists can tell the age and gender of each animal and sometimes estimate to the week in which it was killed.

Thus, Gade said, the lessons to be learned from the site are not just historical but range from ecology to geology, from climate to culture, from anthropology to genetics.

Evidence suggests that right before 1800, some Indian tribes were using a new animal that was revolutionizing their cultures to pull the buffalo carcasses out of the pit: the horse.

Archaeologists also have theorized on at least two major "drive lines" - routes along which tribes directed and then stampeded the buffalo into the hole. One came from the southwest, the other from the north.

The stampedes no doubt used a skillful, carefully planned combination of decoys, noises and visual effects to funnel the herds, Reher said.

"Keep in mind the expertise that these people had who were able to do this. It's an exceptional knowledge that we'll never really understand," he said. "Buffalo are really agile, and they don't just jump off. They can turn around and come right back at you."

At the very least, Gade said, routing the buffalo into the hole "must've been a really exciting day."

"Try to visualize how much of an activity that would've been. Everything was associated with the buffalo, so this was a central event," Gade said. "If you get a feel for that, that's the main cultural story."