All tunnels leak, but this one is a sieve. For most of the last two decades, the Rondout-West Branch tunnel  45 miles long, 13.5 feet wide, up to 1,200 feet below ground and responsible for ferrying half of New York City’s water supply from reservoirs in the Catskill Mountains  has been leaking some 20 million gallons a day. Except recently, when on some days it has lost up to 36 million gallons.

After tiptoeing around the problem for many years, and amid mounting complaints of flooded homes in the Ulster County hamlet of Wawarsing, the city’s Department of Environmental Protection has embarked on a five-year, $240 million project to prepare to fix the tunnel  which includes figuring out how to keep water flowing through New Yorkers’ faucets during the repairs. The most immediate tasks are to fix a valve at the bottom of a 700-foot shaft in Dutchess County so pumps will eventually be able to drain the tunnel, and to ensure that the tunnel does not crack or collapse while it is empty.

For this, the city has enlisted six deep-sea divers who are living for more than a month in a sealed 24-foot tubular pressurized tank complete with showers, a television and a Nerf basketball hoop, breathing air that is 97.5 percent helium and 2.5 percent oxygen, so their high-pitched squeals are all but unintelligible. They leave the tank only to transfer to a diving bell that is lowered 70 stories into the earth, where they work 12-hour shifts, with each man taking a four-hour turn hacking away at concrete to expose the valve.

The other day, one of the divers, A. W. McAfee, moved about as gracefully as anyone could in that much water, slowly fixing a monkey wrench onto a screw wedged in a block of concrete, then taking a mallet to whack it free. As Mr. McAfee did this again and again, a camera on his helmet broadcast his slow ballet and heavy breathing in near-darkness to a video feed far above in a brick building, where his bosses sat, riveted, searching for clues.