(CNN) -- At times with only flashlights to illuminate the way, NYU Langone Medical Center began evacuating about 260 patients, carrying some of them down 15 flights of stairs to ambulances ready to take them to the safety of other hospitals.
The hospital 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 it 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," Brotman said. "The flooding was just unprecedented."
Roof-mounted emergency generators did kick in, but two hours later, about 90% of that power went out, and the hospital decided to evacuate patients.
The evacuation was completed by 11 a.m. Tuesday, according to a hospital statement.
Four 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 had batteries, so those patients did not need manual respiration, he added.
"Anytime you transport any patient at all, let alone a tiny baby ... there are several things that can go wrong," Dr. Sanjay Gupta, CNN's chief medical correspondent, said Tuesday on "The Situation Room" with Wolf Blitzer. "Some of those breathing tubes can be very tenuous, they can come out very easily. Someone obviously manually ventilating, providing air and ventilation to the baby, even if someone gets tired -- that can be a huge problem. Babies breathe faster than adults, so you have to be doing this quite quickly as well.
"But just getting outside in the middle of a storm ... body temperature can change, heart rate, blood pressure," he continued. "So there are a lot of things to monitor even within the hospital. During a transport all that becomes much more difficult."
Some 1,000 staff members -- doctors, nurses, residents and medical students -- along with firefighters and police officers evacuated 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 done by flashlight.
"Everybody's digging in and doing what they have to do," Brotman said, describing the process.
The hospital usually has about 800 patients, but it discharged hundreds over the weekend in anticipation of the storm, he said.
But no one anticipated the high flood levels, or that the generators would get waterlogged.
Kenneth Langone, the chairman of the hospital's board of trustees who also happened to be a patient there until he was discharged Tuesday morning, said that regulations require the generators to be tested regularly and that they've worked every time.
Langone said the hospital is in the midst of an "enormous" building campaign. The generators are going to be replaced in a renovation, he said.
"It's like when you renovate your home, you don't tell the contractor to put the older countertops back in," he said.
During Irene, only one building was flooded, and with just 2 to 3 feet of water. On 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," Brotman said. "That's just an example of how stunning and rapid this flooding situation was."