A huge, granite-eating machine that spent the past eight months chewing a mile-long tunnel beneath a busy Manhattan office district is getting a rest, of sorts, after completing its journey to Grand Central Terminal.

The 200-ton, tunnel-boring rig is one of two that have been carving a pair of new rail tubes that will eventually allow commuter trains from Long Island to connect with Grand Central for the first time.

The massive device, imported from Italy, went into the ground in Queens last spring, crawled beneath the East River through an existing tunnel and began chewing through bedrock in November on the East Side of Manhattan.

It reached its goal deep below Grand Central at the end of June and has since been backing up, slowly, to a new location, where it will begin work on another task related to the project.

The tunnel it made, opened to reporters for a brief tour yesterday, is a marvel.

The tube lies roughly 140 feet below the surface, cut through rock solid enough to support the neighborhood’s many skyscrapers.

Today, it is still a dim, muddy place. Ground water seeps in constantly, trickling down the smooth stone walls and mixing with the slop of gray muck on the tunnel floor.

Construction workers, about 200 a day on the project, arrive via a slow, noisy two-car train that trundles in from Queens.

Conditions inside are warm, wet and “a little nasty,” a construction manager on the project, Sal Calvanico, said.

Occasionally, the lights go out, plunging the sandhogs building the tunnel into darkness.

Tunnel digging can be dangerous work, and there are plenty of reminders underground of the hazards, from official signs bearing safety slogans, to graffiti on the walls that remind workers, in brusque terms, to watch their behinds.

“We control the rock. The rock doesn’t control us,” a veteran sandhog, Dennis O’Neill, said.

There also is dark humor in the job: “How much further,” asks one bit of graffiti, scrawled on the wall on the long train ride to the tunnel’s mouth.

The boring machine is part train, part monster.

With all its trailing gear included, the rig is nearly 360 feet long — big enough to include a tool shop, a small control room, and a break room with a coffee pot and a picnic table.

Its head is a massive contraption that gets leverage by gripping the tunnel walls, then uses 45 steel cutting discs and 6 million pounds of thrust to pulverize anything in its path into muck.

That muck is then loaded onto a conveyor belt that carries it 4 miles, back to Queens, where a second conveyor system hauls it to the surface and into a holding pit. Trucks haul it away at night.

When it is operating at full blast, the operation is a cataclysm of noise and dust, yet it is nearly invisible and inaudible to people on the street.

Even though the tunnel runs beneath landmark hotels such as The Waldorf-Astoria and office towers such as the Seagram Building, tremors from construction rarely reach the surface. Operations at Grand Central and traffic on Park Avenue have continued undisturbed.

Workers eventually will install a concrete lining on the inside of the tunnel to keep the water out. A permanent rail bed will be laid. A new Long Island Rail Road terminal will be built inside a cavern just north of the current Grand Central building.

Blasting on the cavern is expected to begin this month. The Metropolitan Transportation Authority, which owns and operates the rail lines, said the street noise will be similar to the thud of dropping a box of books in another room.

Work on the rail project is expected to continue until 2015.. When complete, it will let 160,000 passengers per day disembark from Long Island Rail Road trains on the East Side of Manhattan, rather than at their current stop at the West Side’s Pennsylvania Station.

In total, the job is expected to cost $7.2 billion.