Boston's Big Dig has sprung a big leak.

Twenty years and $14.6 billion after its conception, the most ambitious public works project in the nation -- an unprecedented effort to route an unsightly highway deep beneath the central city and replace it with a green ribbon of parks -- is dogged by reports of a thousand leaks, cost overruns and faulty waterproofing materials.

Massachusetts Gov. Mitt Romney (R) has said he was "shocked" by the revelations and asked for resignations at the Turnpike Authority, which oversees the project. The state attorney general has opened an investigation. And U.S. Sen. John McCain (R-Ariz.), a longtime critic of the project, has talked about holding a hearing in Washington next week before the Commerce Committee, which he chairs.

The Big Dig's leak problem came to light Sept. 15, when homeward-bound commuters driving deep -- 110 feet down -- in the new northbound tunnel saw an unsettling sight. Saltwater was gushing through a crack in the wall.

"They said millions of gallons were pouring out of that hole," said Eileen Hanley, a schoolteacher who was stuck in the tunnel that night. "I don't like the M-word when I'm stuck in a tunnel."

Work crews trooped in and repaired the hole. But in the weeks since, the Boston Globe has run a series of investigative reports noting that the 95 percent-completed tunnels are plagued by as many as 1,000 leaks and that the waterproofing provided "insufficient protection," according to a 2001 audit report. Engineers with Bechtel/Parsons Brinckerhoff, the project managers, knew of these problems for seven years, according to the audit mentioned in the Globe.

A Bechtel spokesman, Andrew Paven, noted this week that the project is still under construction. Once the above-ground pillars are removed and holes are sealed, he said, the completed tunnel will spring far fewer leaks. Designers projected that the tunnel's drainage system would handle 500,000 gallons of leaking water per year.

Since last December, however, 26 million gallons of leaking water has flowed through the drainage systems.

"Everyone put up with the spiraling costs because they could tell themselves that technologically the tunnel was built right and it was great for the city," said David Luberoff, executive director of Harvard's Rappaport Institute for Greater Boston and a supporter of the project. "But the leaks are unfortunate because it confirms people's worst fears about public projects."

News reports of the leaks have transfixed Bostonians these past few weeks, not least because the Big Dig has dominated the physical city for more than two decades. It is Boston's own municipal ziggurat, a project so grand that it promises to transform what was the staidest of American cities.

Between the turn of the previous century and 1950, Boston changed barely a whit. A single skyscraper pierced its skyline, and few public works projects went forward. Boston was caught between Yankee Brahmin bankers and an Irish Catholic political class, whose distrust of each other was bred in the bone. "Boston had a reputation as a place where public money was 'lost, strayed or stolen,' " said Thomas H. O'Connor, Boston College's university historian. "Not much came our way."

With the end of World War II, Boston faced a choice: It could slip into genteel obsolescence or reinvent itself. Its leaders chose the latter. They set aside grievances and built an elevated highway and tossed up a dozen skyscrapers.

By the 1970s, Boston was slumping again, leaking jobs and cachet. The Brookings Institution examined the future of American cities and ranked Trenton, N.J., ahead of Boston.

Time for Act 3: The Big Dig. As Mayor Kevin White (D) and Gov. Francis W. Sargent (R) conceived it, tearing down the elevated highway would knit together neighborhoods -- the highway stood as a wall separating the Italian North End and the waterfront from the rest of the city. The old highway route would become a 26-acre park, and officials would pour billions more dollars into revitalizing the subway system.

In 1985, officials assigned a $2.5 billion price tag to the Big Dig. By the early 1990s, it had risen to $7.5 billion. It hit the $10 billion mark before 2000. To date, the Big Dig has cost $14.6 billion, an amount that edges higher with each leak.

"They just picked $2.5 billion to get it rolling -- everyone knew that was a lie," said a former state official who was involved in management of the Big Dig and who would not speak on the record because he expects to be questioned by investigators. "Every meeting with the engineers began with the phrase 'unforeseen circumstances.' "

The federal government paid close to 90 percent of the first $6 billion for the Big Dig. Since then, Massachusetts officials have had to divert federal funds from other projects to pay for the tunnel.

Another problem, both Democratic and Republican officials here say, was that former Gov. William F. Weld (R) left too much oversight in the hands of private contractors. When Bechtel officials brought problems to the attention of his administration, the response often was: Go fix it.

"Bechtel of necessity had an enormously powerful role, and Weld abruptly removed three layers of oversight as the project got off the ground," said Fred Salvucci, a former top city and state official. "Weld appointed competent people, but he had an almost libertarian view of privatization and managing a public project."

State officials also ignored court-ordered deadlines for building new subway lines, which had been a condition for approving money for the Big Dig. "There was a lack of political will," said Philip Warburg, president of the Conservation Law Foundation, explaining that the new highways were supposed to be complemented by new subway lines connecting the city's neighborhoods.

All this said, few critics would declare that the Big Dig is a big, wet failure. Five thousand workers labored in dark tunnels; archaeologists, engineers and environmentalists hammered out complex protocols; traffic was rerouted, often on a weekly basis. Yet somehow the city acquired a potentially beautiful facelift without quite grinding to a halt.

"It's been described as doing a triple-bypass surgery while the patient's awake," said Michael Goldman, a radio talk show host here and a political consultant. "It was an amazing feat, as long as we don't end up having to swim to work."

As Goldman's words suggest, the tunnel offers a gold mine of jokes for Bostonians, whose humor often comes serrated. So you walk harborside in South Boston and ask white-haired Tommy Halloran and Bob Norton about the driving downtown, and their responses come rapid fire.

"Fella," Norton advised, "you better outfit your car with pontoons."

What about that harbor tunnel to Logan International Airport? Halloran shakes his head. "I'm not a nervous guy," he said, "but I recommend a wet suit and flippers."

Cars pass a flooded area of the northbound lanes of the Boston tunnel known as the Big Dig. Many leaks have developed in the tunnel.