But the first full-scale plug, made with a single layer of Vectran, failed during a pressure test in 2010. “It ripped right down the middle, from back to front,” Dr. Fortune said.

So Dr. Barbero and ILC came up with a three-layer plug, with the outer layer consisting of woven Vectran belts. It is designed so that the tearing of one belt will not cause a catastrophic failure.

“It took time to figure out how to make it work with such huge pressure,” Dr. Barbero said. “But it also took time for the team to understand how a subway works, what are the constraints in the tunnels.”

A subway tunnel is hardly a pristine environment; it is full of grease and grime — and, often, rats.

“That’s something we’ve talked about,” Dr. Fortune said. “We’ve actually put Vectran samples in tunnels, to see if rats ate it. They didn’t.”

There are also obstructions like tracks, as well an electrified third rail, pipes and safety walkways, all of which could cause gaps between the plug and the tunnel walls. Most of the obstructions can be dealt with by modifying a short section of the tunnel to accommodate the plug, which is 32 feet long when inflated. Sharp corners can be curved, flush tracks of the type used at grade crossings can be installed, the third rail can be discontinued for a stretch, and pipes can be made to swing against the ceiling.

Those modifications will reduce potential gaps but not eliminate them. In the most recent test, when Dr. Barbero and a colleague, Eduardo M. Sosa, inspected the front of the plug, they discovered a two-inch gap in one corner. The procedure called for filling the plug with water to pressurize it further, and then introducing water behind it to simulate a flood. But a plumbing failure, unrelated to the plug, ended the test prematurely. It was repeated successfully several days later, Dr. Fortune said, and the plug held back all but a small amount of water.