In fact, no tunnel worker has been killed on the job since 1997.

For Mr. McCluskey, the tunnel has been his career’s work, spanning 21 years. He keeps a picture from a groundbreaking in Brooklyn in 1993, where he and other workers posed with Mayor David N. Dinkins.

Though the first tunnel may be taken out of service in the coming years, the source of the city’s water itself will not change. The three tunnels all originate at the Hillview Reservoir in Yonkers, near the Bronx border. From the reservoir, the first stage of the third tunnel extends south into the Bronx, then into Upper Manhattan, crossing the Harlem River and going down the West Side of Manhattan before an eastward turn into Central Park. It then crosses the East River into Astoria, Queens. That stage, about 13 miles long, was completed in 1993 and opened in 1998.

By 2006, crews had excavated a tunnel, 12 feet in diameter, down the West Side of Manhattan from Central Park to Canal Street, then into the eastern portion of Lower Manhattan. Four years later, the tunnel had been lined with almost three million cubic feet of concrete.

Image The city has committed $4.7 billion on the project to date. Credit... The New York Times

Outlining the future plans on Wednesday, city officials gathered in the concrete chamber below Central Park; it resembled the bunker of a cinematic supervillain — replete with sump pumps, giant valves and walls pocked with water stains that had aged to a medley of mossy greens.

The setting seemed to rouse Mr. Bloomberg, an engineering major in college, into an expansive mood. He held forth on geometric equations (“Pi r squared,” he reminded reporters, citing a formula for area), the history of the Brooklyn Bridge (“phenomenally dangerous work,” he said) and a 110-year-old, 950-page report in response to a looming water crisis, presented earlier by a deputy mayor.

“I’ll be reading that book later on,” he said.

Mr. Bloomberg has taken a particular interest in the project, noting for years that he has maintained financing even in the face of budget shortfalls.