Out of sight of most New Yorkers is a sprawling underground labyrinth of about 7,500 miles of sewers, part of the city’s vast and, in many cases, aging subterranean infrastructure. Besides old age, the sewers, which are essential to the health of the city, are under assault from a nemesis above ground: grease.

Across the city, the remains of deep-fried this or pan-fried that are being carelessly and improperly poured down kitchen drains and other plumbing outlets. The grease often ends up sticking to other debris in the sewer system, until it hardens and blocks pipes, like clogged arteries.

Carter Strickland, the city’s environmental commissioner, said the cardiovascular analogy was overused — but accurate. “Grease clings to the surface and it builds on itself over time,” Mr. Strickland said. “The sewage cannot get through, and bad things happen.”

Bad things like sewage backing up into sinks and bathtubs, or onto city streets.

“At any one time, it’s such a big network,” Mr. Strickland said, “you’re going to have an issue.”