It requires enough concrete to build a sidewalk from New York to Miami and enough pipe to reach the top of the Empire State Building 140 times over. Workers carved out enough dirt from the ground to fill more than 100,000 dump trucks.

The colossal effort is a water filtration plant being built 10 stories beneath a Bronx driving range, a one-of-a-kind project intended to become a nearly invisible part of the city’s infrastructure.

But the plant has been anything but hidden so far.

The plant’s completion date has been pushed back six years, and its price tag, which early estimates put at $660 million, is now $2.8 billion. Costs, delays, seven-figure fines, and a brush with a high-profile Mafia case have sharpened criticism of the city’s handling of a project that three city watchdog agencies and a group of community leaders are monitoring.

“The bottom line is that to build this water plant, the taxpayers are getting soaked,” a state Assemblyman, Jeffrey Dinowitz, said. “It’s like government at its worst.”

Despite the problems, officials say they will not be deterred from building what they see as the latest far-reaching project in a city full of grand monuments to civic imagination. Officials say they are making good progress despite a late start, and the cost increases are an unavoidable reflection of an industrywide trend.

“The need to complete important projects like the (water) plant has not diminished,” the deputy mayor for operations, Edward Skyler, said. “We can’t sit back and let others worry about the future.”

The federal government has ordered the city to build what will be its first drinking water filtration facility, and the project is believed to be the first subterranean water plant in the nation. Its magnitude is hard to overlook: The pit at Van Cortlandt Park is so deep that large cranes merely peek above the rim.

By 2012, if the schedule holds, a 12-foot-wide tunnel will feed the plant up to 300 million gallons of water a day — about a quarter of the city’s supply. The water will run through a complex set of steps that filter out contaminants: a chemical that makes unwanted particles clump together, air bubbles that push them to the surface to be skimmed off, and a barrier of sand and anthracite coal that strains out still more contaminants. Finally, ultraviolet light will kill bacteria and viruses small enough to have squeezed through the various filters.

New York is one of the few big American cities that doesn’t filter its drinking water, long a point of pride here. It does add chlorine to disinfect its water, fluoride to help prevent tooth decay, and other chemicals that reduce acidity and prevent metals such as lead from leaching from pipes.

Most of the city’s water supply, piped in from rural upstate areas more than 75 miles away, will remain unfiltered. The Bronx plant will treat the 10% to 30% that comes from closer, more suburban reservoirs in what is known as the Croton watershed.

The federal Environmental Protection Agency ordered the city in 1993 to filter the Croton water, saying development near the reservoirs was raising the risk of contamination.

After a 1997 federal lawsuit accused New York of dragging its heels, the city paid a $1 million fine and agreed to build the Croton plant by 2006. A lawsuit over the plant’s location, a wait for needed state legislation and the loss of a key contractor have since extended the completion date to 2012, but the city still rang up another $4.7 million in EPA fines for getting a late start last year.

Meanwhile, the project’s price tag has skyrocketed. So has resentment among critics, who say the project is at best mismanaged, and at worst muddied by building-industry influence. They question whether the city understated the cost of building underground in the city — and not aboveground in the suburbs 20 miles to the north — in the face of heavy lobbying from city-based labor unions and construction industry groups.

“I have never understood how putting a building underground is cheaper than putting a building aboveground,” a local community board chairman, Reverend Richard Gorman, said.

But the city Department of Environmental Protection said the Bronx location provides more security, more city jobs, and will indeed be cheaper to build — largely because it requires shorter tunnels to the water supply.

“All other things being equal, it may be more expensive to have the plant underground than at the surface level, but it wasn’t all other things being equal. The length of tunneling that was required at the respective sites was a significant factor,” the DEP deputy commissioner, Steven Lawitts, said.

The spiraling costs will have an effect on water rates. Mr. Lawitts said the water plant construction accounts for about 7% of water users’ bills, a figure comparable to those for other large-scale projects.

Mr. Lawitts attributes the project’s rising price tag to a widespread rise in construction costs that he says mirrors what is happening across the industry. Critics say those claims are exaggerated.

The city’s Independent Budget Office is examining what factors may have driven up the plant’s price, and the city comptroller is auditing to determine “whether DEP is carrying out construction effectively.” The city Department of Investigation has a full-time monitor at the site.