In the northern reaches of the city of Frederick, the view is of a concrete jumble of discount outlets and parking lots, giving way to an empty cornfield riven by a trash-strewn pit.

To geologist David K. Brezinski, the scene is a mind's-eye mosaic of prehistoric bedrock that once lay 2,000 feet underwater. Scrambling into the pit, he points out the proof: rock outcroppings, their limestone layers scrubbed into relief by groundwater, stair-stepping into a blackened tunnel leading deep -- nobody knows how deep -- into the earth.

It is a sinkhole -- "one of Frederick's biggest," said Brezinski, a Department of Natural Resources geologist who is in an excellent position to know.

For four years, Brezinski has wandered Frederick's fields, riverbanks, roadsides and parking lots carrying a hammer and a notebook-size global positioning device, mapping every sinkhole and potential sinkhole he can find in the Frederick Valley.

The valley, a 250-square-mile basin that cuts through the center of the county, is, according to Brezinski, one of the richest sources of sinkholes in Maryland. The richest of the rich is where he is standing now, midway down the slope of a sinkhole 20 feet deep and half a football field long, in the path of a planned highway extension and acres of proposed development.

"I came here after Jeanne came through," he said, meaning the remnants of the recent hurricane. "It had grown by five meters overnight."

As Frederick contends with the opening of a half-dozen sinkholes in less than a month, Brezinski's report supplies important answers to the questions highway crews, home builders and engineers have asked for decades: Why sinkholes, why here, and where will the next one happen?

Frederick County is underpinned by massive hunks of gray limestone, bashed by prehistoric collisions that folded and fractured it. Groundwater percolates through the fractures, dissolving the rock to create underground chambers through which the water runs, gaining volume and speed.

At the surface, digging and heavy water runoff -- from a road or parking lot -- hasten the process by weakening the chambers' soil cover. Eventually, the surface soil collapses into the chamber. Whatever was sitting on top -- in Frederick, it has been roads, water, communications lines and a car once -- falls in.

His X-ray vision of the region's topography is what makes Brezinski's four-year study, a $500,000 project paid for by his department and the Maryland State Highway Administration, unique. Traditionally, geologists have viewed the region's limestone underpinning, called karst, as composed of one generic brand of limestone. Brezinski, who specializes in stratigraphy, the study of how earth's sedimentary layers form, thought there might be more to it.

Mapping the region, he found 21 subtly different varieties of limestone. Plugging the locations of sinkholes, depressions (which may grow into sinkholes) and karst springs (water emanating from the limestone) into his GPS unit, he discovered far more of them in the type of silty limestone that runs through the central part of the Frederick Valley -- and right through the center of the city.

Using the study, released July 1, "Any prospective home buyer, developer, state or city planner can now identify -- before building or buying -- areas where they should be concerned," Brezinski said.

Downtown, a copy of Brezinski's report and its color-coded topographical map sits in the office of Assistant City Engineer Dan Christy. But Christy hasn't had time to read it: Since mid-September, reports of a half-dozen major sinkholes, abetted by the season's heavy rain, have opened up on city and private property.

The most worrisome by far gaped open Sept. 18 in Riverwalk Park, about 110 feet from a new well the city is testing, a well that officials hope will help remedy the area's chronic water shortage. The hole, about 10 feet across and seven feet deep, had an underground link to the well, tests showed. Another well in the testing phase is less than a half-mile away.

The sinkhole, which has been plugged, has complicated the well testing, which will cost a total of $100,000, and potentially endangers the whole project, Christy said. Usually, for new municipal wells, state regulators require 72 hours of testing, which includes soil sampling, monitoring of the land around the well and water testing during pumping. Frederick's sinkhole problems lengthened the trial period to 60 days. That ends tomorrow, Christy said.

Over the next several days, the city will conduct additional studies on land around the wells in an effort to prove to the Maryland Department of the Environment that the wells are usable.

Department officials "will evaluate our report and determine whether they agree or disagree" with the city's conclusions, Christy said. If they agree, a public hearing will follow.

The city's daily water needs top 11 million gallons, badly taxing its resources. The two new wells, if allowed to pump at the city's target of 500 to 600 gallons a minute, would supply about 7 percent of that total.

Brezinski said his map would have helped predict the problem. Although the wells are surrounded by rock that is less susceptible than some others to sinkholes, they lie near the Monocacy River, in stream lowlands, topography that Brezinski identified as a natural trouble spot.

During the test pumping, "the water table was drawn down, and the soil collapsed into the void the water was filling," Brezinski said. Translation: classic sinkhole.

Giving his rock perch a playful tap with his hammer, Brezinski scrambles out of the sinkhole. Across the fields of concrete, the sun glares on acres of asphalt. During every rainstorm, torrents of runoff water flood into this hole, widening it.

In the distance, highway workers labor on an extension of Interstate 70 that will run straight over this hole. Before they reach it, they'll spend millions of dollars filling it with boulders, concrete and crushed limestone. Then they'll fill the others nearby, some no bigger than a barbecue pit, others deep enough to swallow a car. There are so many, in fact, that highway workers are forbidden to wander here alone.

Not so Brezinski, who strides across the cornfield to his car. And then, in his path, there it is: A new, six-inch-wide hole, twisting its way downward.

Brezinski stops, turns, glances at it. "Gopher hole," he says.

David K. Brezinski of the Maryland Department of Natural Resources stands at the edge of one of the largest sinkholes in the Frederick Valley. Some sinkholes are filled in with boulders, concrete and crushed limestone. The valley, he says, is one of the richest sources of sinkholes in Maryland. Brezinski's color-coded topographical map, noting sinkholes and potential sites, is of particular interest to highway crews, home builders and engineers.