The scale is staggering: Superstorm Sandy flooded New York City's network of underground and vehicular tunnels with up to 400 million gallons of water. On Thursday, the U.S. military began to bring some of its heavy equipment into place – from generators to powerful water pumps to trucks flown from across the country. But, as the U.S. Army Corps of Engineers emphasized to Danger Room, they're just getting started with the cleanup.

The work for the U.S. Army Corps of Engineers in Manhattan starts at the Brooklyn Battery Tunnel, the major underwater thoroughfare for cars passing between Brooklyn and Manhattan. Corps crews assembled at South Ferry, near the Manhattan entrance to the tunnel, have begun receiving power generators and other preliminary equipment by barge from the Navy, ahead of the high-head submersible pumps or centrifugal pumps necessary to start drainage. Task number one is to clear out the underpass to the tunnel, a task that the Corps' Jim Pogue estimates will take about 24 hours, coming from two different sites. The expected extraction: 10 million gallons.

The crews have limited access to the submerged tunnels: There's "only one way in and one way out," Pogue tells Danger Room. Worse, it's a moving target. As the water recedes, "you're chasing it down the tunnel so the pumps have to be continuously repositioned."

Not that the pumps are fully in position yet. The Corps is "looking at bringing in" two types of pumps, a "high-head submersible" and a centrifugal one, Pogue says. The high-head pump goes below the surface, extracting water down from the top, while the pump itself may be submerged as far down as 100 feet. The centrifugal pump is more familiar, using a hose "similar to a straw," as Pogue put it, to suck the water out. The plan is to pump the water back out to sea.

But it's not clear when the pumps will arrive, or how long it'll take the corps to clear out the entire tunnel before moving onto New York's other vehicular tunnels, like the Holland and Lincoln that connect Manhattan to New Jersey. The Corps has already shipped 12 eight-inch pumps and 13 six-inch pumps out to the city – from, of all places, New Orleans. And a Corps team is working with the Navy to get the pumps in place; a Navy spokesman, Lt. Matt Allen, referred comment to the Corps. Pogue added that the Coast Guard is sending 16 pumps to the Tunnel site, from New Jersey, Alabama and California. The Pentagon said late Thursday that it's in the process of moving 120 "high-flow water pumps" to New York and New Jersey.

And not only will the pumps have to be repositioned as the water in the tunnels recede, the Corps crews will have to go slowly as they pump the water out. "These large tunnels take large capacities of water," says Curry Graham, a Corps official in Washington D.C. "We can't de-water quickly because that could cause damage to the structure." (Graham says the Corps hasn't received a request from either the Federal Emergency Management Agency or the City of New York to help bail out the city's warren of subway tunnels.)

It's an "un-watering" process the Corps has had to work on three times since Hurricane Katrina, to the point where the Corps personnel at work on the tunnel underpass come from a special Task Force Unwatering the Corps has based in Illinois. However, the Corps notes, the Task Force "does not have large pumping or construction equipment or operating

manpower on staff or on contract standby; all such assets must be procured under emergency contracting provisions and personnel deployments."

Thousands of Army and Air National Guard troops are already involved in the relief effort, commanded by the governors of their respective states. But the U.S. military is bringing help as well. On Thursday afternoon, five C-5 and 12 C-17 cargo planes are expected to arrive at Stewart Air National Guard base in Newburg, New York, the result of a request from FEMA for military airlift. (See video, above.)

The planes will carry unusual passengers: not military personnel, but a crew of about 12 people and 69 trucks – flatbed diggers, cherry-pickers, mobile command centers and more – from the Southern California Edison electric utility. The utility will assist in a "power restoration mission," said George Little, the Pentagon's chief spokesman, who did not specify where the mission will focus.

Additionally, the Navy is moving three ships to the mid-Atlantic: the amphibious assault ship USS Wasp, the amphibious transport dock San Antonio and the dock landing ship Carter Hall. None of the ships has a designated mission as yet, and Little called their deployment "a precautionary measure." The Wasp is scheduled to reach New York on Thursday, with the other two ships arriving Friday; a Pentagon spokesman, Air Force Lt. Col. Tom Crosson, says they'll float in the waters off New York until the Navy tells them to return to their Norfolk, Va. home port.

The military is only one aspect of the Sandy relief effort. On a Thursday conference call with reporters, the administrator of FEMA, Craig Fugate, said that his agency was shifting away from search and rescue missions and toward power restoration and water clearance missions. Fugate said he did not anticipate using trailers for interim housing assistance, as was the case with Hurricane Katrina. A Red Cross senior vice president on the call, Charley Shimanski, said that his top priority was providing food and shelter over the next two days.

It is unclear what military aid is en route to storm-ravaged New Jersey. The Pentagon said that 2000 New Jersey National Guardsmen were available for Gov. Chris Christie's efforts. Efforts to learn about airlift, cargo or additional help from the state's sprawling Joint Base McGuire-Dix-Lakehurst were unsuccessful on Thursday.

For the Army Corps of Engineers, its priority along the East Coast is and other major transportation systems cleared to operate. The task is huge: the Corps has some 35 "large scale infrastructure projects" in New York alone, Graham says, atop some 26 official assignments from FEMA more broadly. It's got 200 generators ready to provide emergency power at four locations in New York to take some of the strain off the beleaguered Con Edison utility. And the project to clear out the Battery Tunnel underpass is a "smaller and simpler project," Graham says, "while we do the engineering work on the bigger tunnels."