The 300-foot-long tunnel boring machines that will dig the tunnels for the Second Avenue subway, the extension of the no. 7 line, and the Long Island Rail Road connection beneath Grand Central Terminal could be one-hit wonders: They may be abandoned underground when their drilling work is completed.

Abandoning the machines, which cost between $15 million and $20 million apiece, may prove more efficient and cost-effective for project contractors than hauling them out through the holes they carve through soil and rock. If left underground, the machines would be turned away from the tunnels and then retired.

"We could leave it underground," president of the Capital Construction Co. at the Metropolitan Transportation Authority, Mysore Nagaraja, said yesterday, referring to the tunnel boring machine that is scheduled to break ground on the Second Avenue subway project in 2008. The two tunnels that will run along Second Avenue, and perhaps the final resting place of the machine that creates them, will lie about 62 feet below ground, an MTA spokesman said.

"It's a huge assembly. It's not a simple machine, something you can hold. But how the contractor wants to use the machine is really up to him," Mr. Nagaraja said. The contractor for the Second Avenue subway project will be announced today by MTA officials.

The spokesman for the city's Department of Environmental Protection did not return calls yesterday concerning the environmental effects of leaving machinery underground permanently.

The tunnel-boring machine, which drills through rock without destroying the streetscape, is the cleanest method for tunnel excavation. In the past, subway tunnels have been excavated by dynamite blasting, or a method known as "cut-and-cover," in which temporary trenches are dug into streets during construction and then covered up when the project is completed.

Tunnel boring machines were used to dig the 63rd Street tunnel about 30 years ago, where the F and the V lines now pass through. That machine was not abandoned underground because the tunnel was located close to the water, Mr. Nagaraja said.

The machines have also been used to dig subway tunnels in San Francisco, Los Angeles, Portland, Ore., and Washington, but transit officials in Washington and San Francisco said they did not know whether the machines had been left underground when the boring was completed. The six tunnel boring machines used to dig the Channel tunnel, or "Chunnel," a rail link between England and France, were left under the English Channel when the project was completed.

Phase one of construction on the Second Avenue subway line, which will run between 96th and 63rd streets, will employ just one tunnel boring machine. "You have to design the machine to the kind of rock you're dealing with," Mr. Nagaraja said. "This is very hard rock." The two-track line will run between 125th Street and the Financial District.

The tunnel boring machines employed by the MTA for its capital construction projects are manufactured by Robbins, the only American company that designs and makes the machines.