The Thursday before Sandy made landfall, Thomas Creamer, Army Corps of Engineers chief of operations for the New York district, touched base with other districts that might be needed. The storm was not predicted to exceed Category 1 levels. But Creamer's job was to consider all possible options. It was "part of what we were doing just in case something went wrong," he says. "And it went wrong."

Preparing for the worst even when it was not supposed to happen proved key to the Corps' mission, which removed some 500 million gallons of floodwater from New York City infrastructure in less than a fortnight. The Corps dubs the effort "unwatering" rather than "dewatering" because the former more strongly implies the water was not supposed to be in that location to begin with.

Intense collaboration with city agencies, the U.S. Navy, the U.S. Coast Guard and private contractors proved vital as crews scrambled to figure out what kinds of pumps would best serve which sites—and where to acquire those pumps.

"This was a very intense interagency joint mission," says Col. Paul Owen, New York district commander. "On Wednesday before the storm, we worked with the city Office of Emergency Management. We discussed the possibility of the tunnels flooding. We thought it was remote but identified points of contact just in case."

At around 8 p.m. the night Sandy hit, Creamer got a call from former Corps members who were working at the World Trade Center site. "They told us water had begun streaming into the site," he recalls. "As much as you plan for this, you kind of don't believe it will happen."

But Creamer quickly tapped into relationships forged with the U.S. Navy and its on-call salvaging contractor, Donjon Marine Co. Inc. "We had worked with them on getting the USS Intrepid unstuck a few years ago," he says. "We brought them on board Tuesday morning."

Navy Captain Mark Matthews, supervisor of salvage, began scoping out the task. "We ended up doing nine sites. We got a feel for each of the locations—discharge height, volume, what kind of pumps we could use," he says. "Would they be electrically submerged pumps? Hydraulic pumps? Diesel or gas? Each of these questions were driven by the specifics of each of those tunnels."

The Navy called on Donjon Marine. "We got the call before the wind stopped blowing," says John Witte Jr., Donjon's executive vice president. The domino effect continued as Donjon called on several subcontractors to bring in more pumps and people from as far away as Houston.

Centrifugal pumps, which essentially suck water in like a vacuum cleaner, are best suited for conditions with low headroom, Witte adds. Hydraulic submersible pumps "push" water out. "That is 90% of what we used," he says. Twelve-inch pumps prevailed, but smaller pumps were used depending on access and portability.