A few weeks ago, the chairman of the Massachusetts Turnpike Authority descended 100 feet below ground, unfurled a huge American flag, announced the opening of a tunnel, and began scouring history for superlatives. "This project," he boasted, "rivals the Hoover Dam and the Panama Canal."

The project, of course, is the new Underground Central Artery/Tunnel, the $14 billion "Big Dig" that will bury an interstate highway beneath the street and transform the physical character of Boston. The city has been living with the Big Dig's promise, its cost overruns and monumental inconveniences, for the past two decades. As one state legislator said during the opening ceremony, "I was in preschool when this project started."

Boston is famously ambivalent about the Big Dig--it has what one historian calls a love-hate relationship with the project. What everyone seems to accept is that it represents a milestone--a decisive event in the history of American public works.

But what sort of milestone? Is it the end of an era or the beginning- -the last huge piece of infrastructure American government will have the audacity to attempt, or the start of a new period in which big digs--for roads, airports and mass transit--will once again be feasible?

The Turnpike chairman, Matthew Amorello, didn't exactly sound optimistic when he called the Central Artery "one of the great engineering feats of the last century." Amorello seemed to be hinting that the project carried a quality of obsolescence on the very day of its opening. Maybe the Big Dig is a little like Project Apollo: It was incredibly hard, incredibly expensive, and yet we did it. That was great. Now we need to start spending our money on something else.

That isn't the only plausible scenario, however. Within the next couple of years, not only will the underground highway be open in both directions but the ugly elevated Interstate 93 that has defaced and segmented the city for a generation will come down. Open parkland will stretch from the center of town to Boston Harbor. Mayors and planners from everywhere in America will come to see it, and I don't think many will return home calling the Big Dig a waste of money. They will come home wishing they could do something like that themselves.

The larger question of what public works will be like in the aftermath of the Big Dig deserves serious attention from scholars and practitioners. It is now getting that attention from two of the best: Alan Altshuler, who was Massachusetts' transportation secretary when the Central Artery was first conceived; and David Luberoff, who has been chronicling the project from its inception (along with Altshuler) at Harvard's Kennedy School of Government. Just as Amorello was raising the symbolic flag, the two authors were completing an important new book: "Mega-Projects: The Changing Politics of Urban Investment."

As Altshuler and Luberoff see it, there was a Golden Age of mega- projects in the two decades after World War II. These were the years that brought us the Interstate Highway System, dozens of major airports, and billion-dollar urban renewal schemes. They were years in which planners saw the obstacles as largely technological, all but ignoring the social costs of tearing up neighborhoods and displacing residents against their will.

That era began to wind down in the late 1960s, when citizen activism halted the extension of several interstates through urban territory, and suburbanites living near airports learned to block expansion plans by raising environmental objections in sympathetic courts. In 1985, the age of mega-projects came to a symbolic end when New York officials abandoned plans to build Westway, a new highway through Manhattan's West Side. They had failed to satisfy opponents despite redesigning it to run underground, shielding neighborhoods from displacement, and offering billions of dollars to mitigate adverse environmental effects.

It was upon the demise of Westway that the late Senator Daniel Patrick Moynihan issued his famous pronouncement that big projects were no longer feasible at all. "Central Park," Moynihan said, "could not conceivably be built today."

In the years after that, the existence of an "infrastructure crisis" became an article of conventional public policy wisdom. Journalists found it difficult to type the phrase "roads and bridges" without inserting the word "crumbling" in front of it. The familiar acronym "NIMBY" gave way to the even more ominous "BANANA"--Build Absolutely Nothing Anywhere Near Anything.

But as Altshuler and Luberoff demonstrate, it's an oversimplification to assert that nothing important was built during the years of so- called public works stagnation. To a great extent, enthusiasm for mega-projects was simply transferred from roads and airports to economic development projects that aroused less citizen opposition: stadiums, convention centers and festival malls. Compared to large transportation projects, these were benign in their impact on surrounding communities.

And they could usually be built without visible increases in local taxation. Some of the funding came from public-private partnerships, some came from federal Urban Development Action Grants, and some was simply pushed onto the outsiders who would patronize the new facilities. Taxing hotel rooms raised billions of dollars for urban convention centers in the 1980s, and little opposition was ever expressed. People rarely stay in hotels in their own hometowns.

The few large transportation projects that did come to fruition in the stagnant years made it through on the basis of a new tactical approach. They required complex and fragile coalitions that gave numerous interest groups a stake in the project. Transit systems, for example, continued to win approval and survive to completion because highway users and public transit planners joined forces to lobby for what each side wanted. This is why federal funds for transit continued to flow relatively generously even through the Reagan years.

The other tactical switch was to a policy of buying off opponents. Atlanta got new runways at Hartsfield Airport in the late 1970s by settling with 2,500 homeowners, and soundproofing 10,000 homes. Cleveland increased capacity at Hopkins Airport by offering an adjacent suburb half the revenue from a new industrial park that would be part of the project.

Altshuler and Luberoff call this the "do no harm" strategy. A government willing to negotiate with any organized group opposed to a major project could usually get it built in the end, Westway notwithstanding. The problem was that these agreements sent costs soaring. Building Denver International Airport required nearly two decades of "do no harm" bargaining with local activists demanding commitments on noise, air pollution and a share of local tax receipts. When DIA was finally finished in 1994, it was the first major U.S. airport to open in 20 years. It also cost $4.8 billion, nearly three times the initial estimate, due largely to the promises made in the negotiation process.

And thus we come to the Big Dig, the project whose completion seems to belie all the common assumptions about what governments can and cannot do in creating new public infrastructure.

Much has been written about the congressional largesse that underwrote this project: the fact that it qualified (just barely) for inclusion as a piece of the Interstate Highway System, thereby prying loose more than $7 billion in federal funding; the fact that Thomas P. O'Neill Jr., the congressman from Cambridge, was speaker of the U.S. House and saw to it that the money came through despite a veto from President Reagan. It is no accident that the main stretch of Boston's new Central Artery is likely to be named the "Tip O'Neill Tunnel."

But O'Neill aside, the Big Dig required all the methods Altshuler and Luberoff cite as necessary to get anything built in the post-mega- project era. Its mastermind, Fred Salvucci, knitted together a coalition of business, labor, environmental and community groups and kept it together through years of the most intense pressures. The Big Dig is a political as well as engineering masterpiece.

And then, of course, they mitigated like mad. The officials supervising the project signed more than 1,500 separate mitigation agreements, some of which--for amenities such as parkland and new transit lines--had little to do with any tangible harm the Central Artery itself would cause. For the Big Dig, the definition of mitigation was "just about anything that will keep the opposition quiet."

This is the "do no harm" approach that the "Mega-Projects" authors discuss. Of course, some will say a collection of deals that raises the cost of a highway from $2.3 billion to $14 billion is itself a species of harm. That issue is open to argument.

What is not really in dispute is that the Big Dig proves the worst urban planning mistakes of the postwar years are not necessarily irreversible. We can rip them up and start over if we want to do it badly enough. The question is whether that can be done at an acceptable price. No doubt dozens of cities around the country will decide that it can't.

Altshuler and Luberoff, however, seem convinced that some governments will come to the opposite conclusion--that creating what Boston is currently creating justifies spending a great deal of money. "The era of mega-projects," they proclaim," is not over. The game simply has to be played under different rules.

The most important of those rules is that communities that want this sort of physical renewal will need to pay for it. The federal government won't be much help; in the end, even the Big Dig had to settle for 58 percent federal funding, not the original 85 percent. Unlike festival malls and convention centers, transportation projects aren't going to attract massive private investment. And there is a limit to how much pain third parties can be made to accept. Some city hotel taxes now exceed 15 percent. That cash cow is just about out of milk.

And so, Altshuler and Luberoff conclude, any mayor who visits Boston, admires the Big Dig and wishes to emulate it will have do some tough things: debate the project with his constituents, make realistic assessments of its cost, and ask voters to tax themselves to finance it. This may mean that few projects of Big Dig magnitude will get built anytime soon. On the other hand, it is the way American democracy is supposed to work.