Looking across the sere sparseness of this sun-baked knoll, it's hard to believe anyone could live here. But in fact, beneath a farmer's field just a few miles from the Four Corners town of Cortez lies a city.

By today's standards, it wouldn't be much of a city. About 500 inhabitants. Maybe 14 kivas. Most buildings one- or two-stories high.But by prehistoric standards, it was a bustling place. In fact, combined with other nearby ruins, it represents a population larger than the Four Corners area supports today.

The Shields Pueblo, named after the farmer who grew crops here until it became an archaeological dig last year, is a source of intense interest today. It is the most recent dig being conducted under the auspices of the Crow Canyon Archaeological Center.

Crow Canyon is a nonprofit research center, founded in 1968 by Ed Berger and his wife, Joanne, prominent Colorado educators who went to the area to start a school for children who had problems in traditional, mainstream schools.

It evolved into an archaeological study center, because the area is a hotbed of yet-undiscovered or unexplored archaeological sites.

The staff and visitors have excavated various sites with permits from the Bureau of Land Management or the Archaeological Conservancy.

The Crow Canyon excavations are perhaps the most productive sites being studied in the archaeologically rich Four Corners. Within the region's vast landscape are haunting echoes of the Southwest's prehistoric past: Mesa Verde, Ute Mountain, Hovenweep.

What makes Crow Canyon so special is that it's a place where amateurs can roll up their sleeves and work side by side with professionals to experience the hot, dirty, sweaty excitement of making an archaeological find.

"It is one of the largest unexcavated archaeological sites in the region," says Guy Prouty, project director for Crow Canyon Archeological Center.

Yet none of it is readily visible.

Some adjacent ruins, partly visible above the surface, are hidden by scrub oak in the nearby gully called Goodman Point.

The visible ruins of the Shields Pueblo were bulldozed by the farmer, who got tired of plowing around them. The rest remain below the surface of the field. Only the deep dimples in the field give away the fact that structures (in this case, kivas) are buried below.

How does one unearth a city?

With lots of volunteer help.

"Volunteers probably do 99 percent of the excavating," Prouty says.

The volunteers go to Crow Canyon on vacation and pay about $700 a week to work there.

Pay to work in the hot sun, performing tedious tasks?

Yes, and gladly, it seems.

They get housing, meals and a thorough education in archaeology.

Sound like a lot of work? It is. But it's so fascinating, the project has a 40 percent return rate among visitors.

Last summer, for example, Cindy Flosman from Anaheim, Calif., was visiting for her third time.

"I love it here," she says. "It's just so much fun."

For 30 years, Crow Canyon (a project name, not a specific place) has been offering outdoor education. For 15 years, it has been using volunteers to do the work of discovering the remains of ancient civilizations, said marketing director Shannon Gallagher.

"Southwestern archaeology and Indian culture have been largely ignored by history books," Gallagher says. "This is a new learning experience for many people who come here."

Local Indians often serve as advisers to the professional staff.

"They help us decide everything from the politically correct way to word a press release to where to dig," she says.

A typical week for a volunteer goes something like this:

Sunday: A new group of volunteers arrives at Crow Canyon. They settle into their hogans, a communal living situation divided by gender. Men and women have separate bathhouses. Everyone quickly gets to know one another.

Monday: There is a hands-on exercise where newcomers learn about artifacts and what specifically is being studied at the Shields excavation site. Then there is a tour of other excavation sites, past and present.

Tuesday: Most of the day is spent in the lab, so volunteers can see what it is they're looking for and how it's handled once it's tagged.

"A lot of discoveries are made not in the field, but in the lab," Gallagher says.

Wednesday: Volunteers spend a full day at the field site, excavating a 1-by-1-by-1-meter square hole in the earth. They do this not with a shovel but with a trowel and a brush.

"In a lot of ways, we're like Tom Sawyer getting a bunch of Huck Finns to help us paint the fence," Gallagher says.

Thursday: Volunteers spend another day in the field excavating. They also can take an ecology hike.

Friday: Some opt for a behind-the-scenes tour of nearby Mesa Verde National Park's famous ruins. Others can't stop digging.

"If they have been bitten by the digging bug, they can keep digging," Gallagher says. "It's very addictive."

Friday evening: There is an informal party with a graduation ceremony and a run-down of the week's accomplishments.

Saturday morning: Everyone leaves, "usually reluctantly," Gallagher says.

It was volunteers who excavated Sand Canyon Pueblo, another nearby major site, from 1983 to 1993.

Researchers discovered that the area was inhabited from A.D. 1250 to 1280, and had 400 rooms, 100 kivas and 14 towers.

But volunteers and professional archaeologists still are seeking answers to many questions about these sites: What the climate might have been like at the time of habitation. What was the social organization of the Indians who lived there. What arts and cultural pursuits the residents had. If they were involved in warfare. And - the big question - why they ultimately abandoned the place.

"The Indians tell us that it was no single cataclysmic event that caused the ancestral Pueblos (sometimes called Anasazi) to leave," Gallagher says. "They feel it was more of a pull to migrate, a `Honey, I think it's time to move' feeling more than anything else.

"There may have been many reasons - drought, soil depletion, the build-up of trash, a threat from an enemy," she says.

By having a year-round, on-site staff, the Crow Canyon project has the advantage of staying in one place, year after year, always pursuing the next step of the dig.

Visitors shouldn't expect to see another Mesa Verde here.

After each part of a site is excavated and examined, it is back-filled to protect it.

The Crow Canyon philosophy is, "It's not so much what you find; it's what you find out," Gallagher says. "We're not actually after the artifacts so much as the information they give us."

Each year, about 4,000 volunteers - half adults, half children - help them uncover the mysteries of the ancestral Pueblo tribes who lived in the area. They come as individuals, or in groups. There are sixth-graders and retirees.

The children are often as excited as the adults, and they are great workers, Gallagher says.

"They're young and healthy and boy, can they move a ton of dirt! We love them!" she says.

Volunteers also are significant contributors to the Crow Canyon budget. Their tuition pays for the ongoing archaeological work.

It takes $3 million a year to run the operation, which has 60 employees when fully staffed.