So much to think about down here in the biggest hole ever dug in Washington--the river that flowed maybe 2 million years ago, the tobacco that grew two centuries ago, the 1861 Prussian pfennig that went astray, the 19th and 20th century riots, the bottle of Mrs. Winslow's Soothing Syrup that someone quaffed (did it help?), the fickle Sassafras dirt market these days . . . and how much farther to China?

But nearly 65 feet below the surface of Shaw, Michael Caiazzo keeps his imagination in check. He concentrates on creating void out of form. In jeans, plaid shirt and shades, he's at the controls of a great yellow backhoe. He pirouettes the carriage on its treads and whips around the elbowed arm with a bucketful of earth to drop into a dump truck.

Caiazzo can dig dirt like spooning sugar. It takes him 90 seconds to fill a truck, and the trucks can hardly keep up. The empty ones approach from the right and depart, brimming, to the left. Sometimes, the steel arm waits in impatient praying mantis suspension, holding the bucket curled under with a load of dirt until an empty truck arrives.

Ask him about his technique and Caiazzo, 57, will shrug and wrap his cracked and weathered hands around the joysticks in the cab and say, "You just have to know what to do with these."

A man of few words, and not easily impressed, he will add: "It's quite a job. I was amazed when I saw the site."

Amazed, because it is so huge. Unlike New York, which springs up when it builds big, Washington digs down. The new Washington Convention Center at Mount Vernon Square is no exception. Residents insisted that the massive edifice not dwarf their dwellings. The builders are placing the main exhibition hall underground, with the floor 45 to 65 feet below the sloping surface. The District's largest finished basement will extend five square city blocks.

For now, the site looks like ground zero in a Hollywood disaster epic titled "Attack of the Amazingly Rectangular Asteroid." Between Mount Vernon Square and N Street NW, and between Seventh Street and Ninth Street NW, is a canyon, with small people and toy trucks way down there working. Around the rim, construction trailers are perched like cliff-side condominiums.

The digging began in March and is planned to be largely completed by April, followed by construction. The $685 million convention center is scheduled to open in 2003.

Holes this big are measured in units of absence:

* 1.3 million cubic yards of dirt will be excavated.

* 130,000 dump truck loads will be needed to haul the dirt. If that many trucks were placed bumper to bumper, the line would stretch from Washington to Atlanta.

* The hole will reach 11 feet below the water table. Without an elaborate pumping system, it would be a lake. Every day, 70,000 gallons of water are pumped from the site into the city's storm drains. The D.C. Water and Sewer Authority says that is not enough to strain the system.

As anyone knows who has built a modestly ambitious sand castle, digging a hole involves science and art. Caiazzo is at the earth-scraping edge of a tightly choreographed performance of about 150 people engaged in everything that has to do with moving dirt.

"It requires a lot of planning," says Gregory S. Colevas, the project executive for Clark/Smoot, the joint venture of Clark Construction and Smoot Construction that is building the center, with a legion of subcontractors.

Colevas, 37, whose last big assignment was building the stadium now called FedEx Field in Landover, wears a hard hat and boots with his business attire when he crosses the street from his temporary offices in the Carnegie Library to visit the site. Along with his lieutenants, senior project manager Granger A. Stuck and senior superintendent Ronald D. Strompf, Colevas must worry about such details as what mounds of dirt to excavate in what order.

Digging a deep hole is a journey back through time, disrupting the accumulation of ages to create space for a future.

The soft ground where Colevas stood watching Caiazzo one afternoon was deposited there by the ancient Potomac River when its course cut through what is now Shaw. Colevas was way over his head in the Wicomico Formation, a terraced geological structure built of river sediments within about the past 2 million years, said Will Logan, assistant professor of geology at George Washington University.

Look how strange and changeable is the soil. Walk a few feet in any direction, and different stuff will be beneath your feet, from clay to sand and back, with detours into gravel and loam and silt. The floor of the N Street end of the hole, 65 feet down, is like a beach, with sparkling white sand. Soil scientists call the dirt a mix of Beltsville, Chillum and Sassafras complexes, a we-can't-make-up-our-minds description of closely related grades of granular, sandy, silty, loamy dirt.

After the geologists came the archaeologists. They dug test trenches all over the site for traces of human activity.

Tobacco grew at the time of the Revolutionary War. Houses and shops were built in the mid-1800s, and a mix of middle-class blacks, whites and recent immigrants lived in the area for 100 years. In 1857, the Plug Uglies gang from Baltimore rioted here on Election Day in a failed attempt to block the election of immigrants naturalized as citizens. By 1968, the area was economically depressed, and many of the buildings were burned in the riots after the assassination of the Rev. Martin Luther King Jr. Most of the land became a vast parking lot.

The archaeologists were disappointed. The recent destruction and paving had disrupted most vestiges of historic life. They found the pfennig, some clay pipes, a silver spoon, the soothing syrup and a broken bottle of ancient salad dressing.

Next came the backhoe operators, and serious digging began March 8. One of the first orders of business was to install the slurry wall--the main defense against a cave-in burying everybody alive.

A clam bucket, capable of digging straight down, scooped out a 70-foot-deep trench around the site. The trench was filled with slurry, a soupy polymer-water mixture. Then a steel cage was lowered into the trench, and concrete was poured in, decanting the slurry. The result: a buried three-foot-thick wall around the site.

Then diggers like Caiazzo went to work. They scraped away layers and layers of Beltsville, Chillum and Sassafras, exposing more and more of the wall.

But the pale gray wall was not expected to stand on its own. To brace it, thousands of braided steel cables were inserted through the wall and plunged at least 70 feet into the ground behind the wall. Cement grout was injected into the cable holes to anchor them. These tiebacks bound the wall to the surrounding earth.

Then the excavators could dig deeper. "It takes a while before you know how to make the water drain," said Gloria Hammond, 37, whose assignments include maintaining the truck paths in the hole with her loader. "I feel I'm a part of history."

The diggers are employed by Cherry Hill Construction of Jessup, which has the excavation subcontract. It is also Cherry Hill's job to dispose of the dirt.

Jim Openshaw, president of Cherry Hill and a leading dirt merchant, likens this part of the business to a high-stakes game of skill and luck. Dirt is a commodity, but it is perishable: When Openshaw's trucks haul a load from the site, they must have an immediate customer willing to pay for it, or Openshaw loses. If no one wants it, he must pay $35 a load to dump it somewhere, such as an old gravel pit in Oxon Hill.

Who would buy dirt? Many construction projects around the region need dirt as backfill. Every morning, Openshaw checks the spot market for dirt demand.

It's a tricky business. The convention center hole has turned out to have at least twice as much soil contaminated with petroleum from old gas stations as expected. Clark/Smoot's contract protects it from losses because of contamination, but the added cost to the Washington Convention Center Authority is at least $4 million, and Cherry Hill will suffer because there is less usable dirt than Openshaw anticipated.

The trucks and the digging are considered a nuisance by some neighbors. Other people pause at the Mount Vernon Square Metro station and look down on the hole that changes daily, briefly mesmerized.

Sixty years ago, Virginia Lee Burton wrote "Mike Mulligan and His Steam Shovel," the classic children's book that defined the enduring fascination with diggers. Mulligan was excavating a big cellar for the new town hall. With his steam shovel, he could do the work of 100 men, and he made his corners neat and square, just like Caiazzo.

But Mike forgot to leave a dirt ramp to get out of the hole. He was stuck.

"The last thing you pull out of the hole is the ramp," said Openshaw.

Come spring, Caiazzo and the others will drive their backhoes and loaders one more time up the dirt ramp. Then a special excavator with a long arm will reach from the surface into the hole and scoop out the last of the Beltsville, Chillum and Sassafras.

CAPTION: Site foreman Rodney McGraw stands watch over his area of the new Washington Convention Center construction site in Shaw. The excavation covers five city blocks.

CAPTION: Workers building flooring wait for concrete to be lowered to the bottom of the Convention Center site in Shaw.