The agency then sends another crew out to do more detective work, sometimes with electronic transducers that can pinpoint the location of a leak.

The crews are busy this time of year because this is the season for water main breaks. The city says 44 percent of water main breaks happen in December, January and February, when temperatures chill pipes at night and warm them by day. Valves and joints expand and contract. The motion can contribute to leaks and breaks.

“The freeze-thaw cycle certainly plays a part in causing breaks,” said Mr. Sapienza, the environmental protection commissioner. “Winter is always tough, and not necessarily if it’s cold for a long period of time. If you have 20 degrees one day and 45 degrees the next day, that presents a challenge.”

The environmental protection department has recorded more than 400 water main breaks a year nearly every year since 1998.

But despite the number of breaks and how tightly packed things are under the streets, it is relatively rare for the subway to be upended when something does go wrong.

Aside from the two water main breaks in January that caused major delays, only twice in all of last year and once in 2018 was service affected by a ruptured pipe, according to figures from the Metropolitan Transportation Authority, the state-controlled agency that runs the subways.

One of the breaks last month sent waves crashing into the steps of Lincoln Center and disrupted subway service along a stretch of the Nos. 1, 2 and 3 lines.