(CNN) -- At times with only flashlights to illuminate the way, NYU Langone Medical Center was evacuating some 260 patients, carrying some of them down 15 flights of stairs to awaiting ambulances ready to take them to the safety of other hospitals.
NYU didn't anticipate such heavy flooding from Sandy, the superstorm that hit Monday, and chose not to evacuate all its patients before the storm, as they did with Hurricane Irene a year ago. But between 7 and 7:45 p.m. Monday, the hospital's basement, lower floors, and elevator shafts filled with 10 to 12 feet of water, and the hospital lost its power, according to Dr. Andrew Brotman, senior vice president and vice dean for clinical affairs and strategy.
"Things went downhill very, very rapidly and very unexpectedly," he said. "The flooding was just unprecedented."
Emergency generators did kick in, but two hours later, about 90% of that power went out, and the hospital decided to evacuate.
By 1:30 a.m., about half the patients had been evacuated, including all the patients in the adult, pediatric, and newborn intensive care units. Brotman said he anticipated the evacuation would last until around 6:30 a.m.
Water pours over cars in Manhattan as superstorm Sandy flooded large parts of the island on Monday, October 29. 2012. The storm surge from Sandy set records in lower Manhattan, where flooded substations caused a widespread power outage. View photos of New York bracing for impact.
Taxis drive down a New York street where the power was out late Monday, October 29.
Rising water rushes into an underground parking garage in New York's financial district on Monday.
A firefighter speaks to a colleague while surveying damage caused by Sandy on Monday in New York.
Flooded cars line the streets of New York's financial district Monday night.
A truck drives by a flooded gas station in the Gowanus section of Brooklyn on Monday.
A flooded street is seen at nightfall during the storm on Monday in Atlantic City, New Jersey.
Firefighters evaluate an apartment building in New York that had the front wall collapse during the storm on Monday.
Much of FDR Drive in New York was flooded by Sandy.
Heavy rains fall in Manhattan on Monday.
People walk through water on the beach near high tide Monday as Sandy approaches Atlantic City.
Two men run down Foster Avenue while dodging high winds and waves from the storm on Monday in Marshfield, Massachusetts.
An emergency vehicle plows through floodwaters on Monday in Dewey Beach, Delaware.
A person tries to cross the street during the storm on Monday in Atlantic City.
A traffic sign warns motorists west of Philadelphia on Monday.
A wall of water makes its way to shore as residents brave the storm Monday in Ocean City, Maryland.
A downed tree and fallen power lines lie over homes Monday on Harvard Street in Garden City, New York.
Two people shoot video along Brooklyn Heights' Promenade on Monday as Sandy approaches landfall.
Work crews push sand from a roadway in Fort Lauderdale, Florida, due to storm surge related to flooding on Monday.
Two women battle wind and rain with umbrellas in hand in Philadelphia on Monday.
Kira Brizill leads family members as high tide and winds flood the street on Monday in Freeport, New York.
John Edgecombe II, who is homeless, takes refuge from the rain and wind at a bus stop in Ward Circle in Washington on Monday.
Superstorm Sandy dumped a lot of rain, flooding a part of Greenpoint, Brooklyn
A Pennsylvania Department of Transportation truck slowly drives on the Pennsylvania Turnpike as Sandy approaches Bensalem, Pennsylvania, on Monday.
Buses at Frankford terminal in Philadelphia, Pennsylvania, sit idle after Mayor Michael Nutter ordered that all city offices be closed Monday and Tuesday due to potential damage from Sandy.
A woman walks down the promenade along the East River in New York City on Monday.
Jillian Webb, left, and Arianna Corso are pelted by wind and sand on Lighthouse Beach in Chatham, Massachusetts, on Monday.
Waves slam into the sea wall in Scituate, Massachusetts, on Monday.
Chris Losordo carries his father, Vin, across a flooded road in Falmouth, Massachusetts, on Monday.
A repair truck drives down a flooded street in Rehoboth Beach, Delaware, on Monday.
Superstorm Sandy dumped a lot of rain on West Side Highway in Manhattan, NY.
Floodwaters cover the streets of Rehoboth Beach, Delaware, on Monday.
Multiple waves hit the Cooper's Beach in Southampton, N.Y.
Waves crash against a previously damaged pier in Atlantic City, New Jersey, as Hurricane Sandy approaches landfall on Monday.
High winds broke part of a crane boom on this building under construction in Manhattan, causing several nearby buildings to be evacuated.
An emergency vehicle drives down Cape May, New Jersey's flooded Ocean Avenue on Monday.
A young boy runs along Rockaway Beach in the Queens, New York, on Monday.
A woman examines her storm-damaged porch as heavy rain continues to pour in Winthrop, Massachusetts, on Monday.
A lone figure makes his way down Seventh Street in Lindenhurst, New York, on Monday.
People brave high winds and waves in Winthrop, Massachusetts, as Hurricane Sandy moves up the coast on Monday.
A tree felled by the storm blocks Kramer Drive in Lindenhurst, New York, on Monday.
Waves crash over a street in Winthrop, Massachusetts, as Hurricane Sandy comes up the coast on Monday.
A police vehicle drives through a flooded area in New York on Monday.
The New York skyline is seen from the bank of the East River on Monday.
People walk on the boardwalk in Ocean City, Maryland, on Monday.
A man stands on the beach as heavy waves pound the shoreline Monday in Cape May, New Jersey.
The dome of the U.S. Capitol building is seen through a window as heavy rain hits Washington on Monday.
A member of the press takes a photo of a flooded street on Monday in Atlantic City, New Jersey.
A man takes a picture of the storm with his phone from the boardwalk in Ocean City, Maryland, on Monday.
A man stands on the sidewalk Monday as a vehicle drives up a flooded street in Atlantic City.
The Hudson River comes over the sea wall along the West Side Promenade in the Battery Park area in New York on Monday.
The owner of the Wilton House locks up his bar on Monday in Hoboken, New Jersey, as Hurricane Sandy approaches the area.
Two people stand near the edge of the boardwalk on Monday in Ocean City, Maryland.
People fight against the wind along Brighton Beach in New York on Monday.
A jogger runs along the East River in New York on Monday as a police car secures the area.
A man watches as the tidal surge pounds a pier in Ocean City, Maryland, on Monday.
A street on the shoreline of Milford, Connecticut, floods at high tide as Hurricane Sandy approaches on Monday.
A sailboat smashes on the rocks after breaking free from its mooring on City Island, New York, on Monday.
A lone tourist stands in Times Square early Monday as New Yorkers brace against Hurricane Sandy.
A satellite image taken at 12:25 p.m. ET Monday shows Sandy moving over the Northeast.
A restaurant on the boardwalk in Atlantic City, New Jersey, is boarded up in preparation for the bad weather on Monday.
A man walks down a flooded street in Atlantic City on Monday before the hurricane makes landfall.
Tourists wear plastic ponchos in Times Square on Monday.
Air Force One arrives at Andrews Air Force Base in Maryland. President Barack Obama canceled his appearance at a campaign rally in Orlando, Florida, and returned to Washington to monitor the response to Hurricane Sandy.
A road leading to casinos in Atlantic City is empty before the hurricane makes landfall on Monday.
Obama steps off Air Force One on Monday after arriving at Andrews Air Force Base.
A truck moves north on South Long Beach Avenue as rising water and wind ahead of Hurricane Sandy flood the area on Monday in Freeport, New York. The storm, which threatens 50 million people in the eastern third of the United States, is expected to bring days of rain, high wind and, in places, heavy snow.
An overhead sign on the Southern Parkway alerts motorists to road closings in Wantagh, New York, on Monday.
A truck fights its way through water on a road in Southampton, New York, on Monday.
Andy Becica watches the heavy surf from Hurricane Sandy wash in Monday at Cape May, New Jersey. The full force of Hurricane Sandy is expected to hit the New Jersey coastline later Monday.
Water forced ashore ahead of the hurricane starts to flood Beach Avenue in Cape May on Monday morning.
A tattered piece of a billboard blows in the wind Monday in Atlantic City, New Jersey.
Water floods a street in Atlantic City.
An ambulance maneuvers through water on Rockaway Beach Boulevard in Queens as the weather sours Monday in New York City.
People pose for pictures on the Brooklyn Bridge on Monday.
A wave crashes over the bow of a tugboat in New York Harbor on Monday.
Cape May Lighthouse shines over the heavy surf.
Dark clouds cover the skyline of Manhattan early Monday.
A satellite image shows Hurricane Sandy at 8:25 a.m. ET Monday. Forecasters warned that Sandy was likely to collide with a cold front and spawn a "superstorm" that could generate flash floods, snowstorms and massive power outages.
People stand on the beach watching the heavy surf caused by the approaching hurricane on Sunday in Cape May.
Sean Doyle of Levittown and Andrew Hodgson of Hicksville pull their boat from Long Island Sound on Sunday at Oyster Bay, New York.
With Hurricane Sandy approaching, the Long Island Railroad announced the suspension of service at 7 p.m. Sunday in Hicksville, New York.
Lisa Cellucci holds her umbrella as it is blown backward by Hurricane Sandy's winds as her friend Kim Vo watches on Sunday in Cape May.
People look at the surf as high winds and heavy rain from Hurricane Sandy arrive in Virginia Beach, Virginia, on Sunday.
A construction worker covers air vents Sunday to try to prevent the New York subway system from flooding by Hurricane Sandy. New York Gov. Andrew Cuomo announced a shutdown and suspension of all subway, bus and commuter rail service in response to the storm.
Residents of Long Beach, New York, fill sandbags on Sunday in preparation for the storm.
A satellite image from 10:10 a.m. ET on Sunday shows Hurricane Sandy in the Atlantic Ocean grazing the East Coast.
A man surfs at Rockaway Beach in Queens as Hurricane Sandy approaches Sunday.
Scott Davenport brings plywood to cover the windows at the Trump Plaza casino on the boardwalk in Atlantic City, New Jersey, on Sunday.
Bob Kaege takes a measurement while boarding up a shop in Cold Spring, New Jersey, on Saturday as Marie Jadick speaks on the telephone getting an updated weather report in preparation for Hurricane Sandy.
Houses are flooded in the neighborhood of La Javilla in Santo Domingo, the capital of Dominican Republic, on Friday.
Residents watch firefighters battle a blaze in Kingston, Jamaica, on Friday. The fire, which destroyed the home, was started by a faulty generator that was triggered when Sandy caused a blackout, firefighters said.
A motorcyclist rides through a flooded street Friday in Petit-Goâve, Haiti, where three overflowing rivers put homes and farms under water.
Corey Hutterli works on securing his sailboat as the outer bands of Hurricane Sandy are felt in Miami Beach, Florida, on Thursday, October 25.
A woman stands at the entrance of her house surrounded by flood water after heavy rain in Santo Domingo on Thursday.
People walk on a flooded street after Hurricane Sandy hit Port-au-Prince, Haiti, on Thursday.
Burt Myrich boards up a home in preparation for Hurricane Sandy on Saturday in Cape May, New Jersey.
A woman peers out the door of her house Thursday after it was damaged by Hurricane Sandy in Bayamo, Cuba.
A man clears debris from his house on Thursday. It was demolished by Hurricane Sandy in Santiago de Cuba.
Residents in Bayamo, Cuba, try to fix a house damaged by hurricane Sandy on Thursday.
A U.N. peacekeeper on Thursday stands at the edge of a bridge that was washed away by heavy rains from Hurricane Sandy in Port-au-Prince, Haiti.
A house ruined by heavy flooding from Hurricane Sandy sits abandoned in Port-au-Prince, Haiti, on Thursday.
Men deal with downed tree branches after heavy rain caused by Hurricane Sandy in Kingston, Jamaica, on Wednesday, October 24.
Students walk in floodwater from Hurricane Sandy's rain in Santo Domingo on Wednesday.
Citizens of Bayamo, Cuba, buy food on Wednesday, as they prepare for the arrival of Hurricane Sandy.
Waves hit the coast in Santo Domingo on Wednesday.
Citizens of Bayamo talk on the sidewalk on Wednesday.
People in Bayamo hold umbrellas as they purchase food Wednesday before the arrival of the hurricane.
Jamaicans shelter themselves from the rain of approaching Hurricane Sandy as they walk along the Hope River on Wednesday.
The Hope River begins to swell with rain from approaching Hurricane Sandy in Kingston on Wednesday.
Houses sit along the Hope River in Kingston on Wednesday.
A satellite view shows Hurricane Sandy's position on Wednesday.
Photos: Sandy's destructive path
High winds from Sandy knock out power
Sandy leaves many in the dark
Firefighters use boats to save residents
iReporters share Sandy images
Four of the newborns were on respirators that were breathing for them, and when the power went out, each baby was carried down nine flights of stairs while a nurse manually squeezed a bag to deliver air to the baby's lungs.
"This is a labor intensive, extremely difficult process," Brotman said.
The adult respirators have batteries, so those patients have not needed manual respiration, he added.
Some 1,000 staff members -- doctors, nurses, residents, and medical students -- along with firefighters and police officers are evacuating the patients. Since about 10% of the backup power was working, there were a few lights on in the hallways. But still much of the work was being done by flashlight.
"Everybody's digging in and doing what they have to do," Brotman said.
The hospital usually has about 800 patients, but discharged hundreds of patients over the weekend in anticipation of the storm, he said.
But no one anticipated the high flood levels, or that the generators, which are on top of the hospital, would get waterlogged.
During Irene, only one building was flooded, and with just two to three feet of water. Monday night, seven hospital buildings were flooded with 10 to 12 feet of water each, including the medical school and the Smilow Research Center, which was built about three years ago.
"It had a very sophisticated foundation that was built specifically to withstand a flood, but it flooded anyway," he said. "That's just an example of how stunning and rapid this flooding situation was."