Story highlights "We've never seen anything like this," a hospital official says

National Guard troops carried patients out of the hospital, he says

After the storm, troops formed a "bucket brigade" to carry fuel to generators

The hospital is near another hospital forced to evacuate patients after the storm

They called them the "human bucket brigade."

National Guard troops lined up in stairwells at Bellevue Hospital, carrying oil for generators up 13 flights of stairs. Superstorm Sandy had knocked out the hospital's power, and flooding had wiped out basement fuel pumps designed to power its generators.

On Wednesday, when officials at one of the country's largest hospitals decided the storm damage was too extensive to keep the facility running, those same troops helped carry patients down flights of stairs to waiting ambulances, said Alan Aviles, president of New York City Health and Hospitals Corporation.

The troops were "tireless," he said, thanking them for their efforts.

More than half of the 725 patients who were inside Bellevue when the storm hit have been transferred, Aviles told CNN on Wednesday evening. The remaining 260 patients will be evacuated by Thursday, he said.

Dozens of ambulances lined up outside the hospital to carry patients to other facilities.

Photos: Photos: New York recovers from Sandy Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Friends and members of the Puglia family sift through the remains of their missing home for valuables on November 6, 2012, after Hurricane Sandy hit Staten Island, New York. View photos of New York preparing for Sandy. Hide Caption 1 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A part of a house that floated off its foundation during the storm now sits in the middle of a coastal estuary in the Oakwood Beach area of Staten Island. Hide Caption 2 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – New York City police officers and a resident warm up in front of a fire in a blacked-out area of Oakwood Beach on Staten Island. Hide Caption 3 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A woman looks through a sand covered parking lot used as a donation center on November 5 in the Rockaway neighborhood of the Queens borough of New York City. Hide Caption 4 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Volunteers who call themselves the Broad Channel Police Department help clean a neighbor's damaged house. Hide Caption 5 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – People wait in line for food at a distribution center at Coney Island. Hide Caption 6 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A dump truck empties trash collected from homes damaged by Superstorm Sandy on Saturday, November 3, in the Midland Beach neighborhood of Staten Island, New York. New York is trying to clean up and resume normal activities days after the storm hit. Hide Caption 7 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A volunteer places donated clothing in plastic bags to protect it from potential rain at a relief center on Saturday in Midland Beach. Hide Caption 8 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A damaged chair sits on the beach in low-lying Coney Island on Saturday. Hide Caption 9 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A man salvages a set of drums at Rockaway Beach in Queens, New York, on Saturday. Most of the Rockaway Peninsula remains without power. Hide Caption 10 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – People gather among debris from Superstorm Sandy and boxes of donated goods on Saturday in Rockaway Beach. Hide Caption 11 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Boats that were swept aground during Sandy sit against homes near a marina on Staten Island on Friday, November 2. Hide Caption 12 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Eddie Liu uses a broom to clean up mud and water from a flooded coin laundry in Coney Island on Friday. Hide Caption 13 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Michelle Dumas hugs her daughter Olivia to warm her up as they wait in an hours-long line for gas at a station Friday on Staten Island. Hide Caption 14 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Men try to clean up the destruction in a flooded deli in Brooklyn, New York, on Friday. Hide Caption 15 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Members of the Traina family on Friday sort through photographs and other personal items from their Staten Island home that was destroyed by Superstorm Sandy. Hide Caption 16 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – In the aftermath of Superstorm Sandy, replacement power utility poles are transported along Old Country Road on Friday in Plainview, New York. Hide Caption 17 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Abandoned and flooded cars are piled up on Friday, November 2, in the heavily damaged Rockaway neighborhood, in Queens, where a large section of a landmark boardwalk was washed away. Hide Caption 18 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Food trucks and men selling bread and cheeses line Broadway at Union Square on Friday in New York as the city recovers from the effects of Superstorm Sandy. Hide Caption 19 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Debris piles up near the foundations of the iconic boardwalk, which was washed away in the Rockaway neighborhood in Queens. Hide Caption 20 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A man rides his bike through the heavily damaged Rockaway neighborhood. Hide Caption 21 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Matthew Mark Meyer, left, and Annie Barwick, center, sit in a darkened Randolf Beer Restaurant which is using generator power to reopen on Friday in New York City. Hide Caption 22 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – People walk through the heavily damaged Rockaway neighborhood. Hide Caption 23 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Commuters pack into a train on Thursday in New York City. Limited public transit has returned to the city, where 14 of 23 subway lines are running. Hide Caption 24 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Commuters ride the subway. Public transit is operating in New York City, but travel times are long, up to five hours in some cases. Hide Caption 25 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A police officer directs passengers waiting on Thursday to board city buses into Manhattan at the Barclays Center in Brooklyn. About 4,000 buses are replacing the subway lines still closed by Superstorm Sandy damage. Hide Caption 26 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Thousands of people wait to board city buses into Manhattan, and some subway lines remain underwater. Getting water out of the tunnels is "one of the main orders of business right now," Gov. Andrew Cuomo said Thursday. Hide Caption 27 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A police officer directs traffic entering the Brooklyn Queens Expressway into Manhattan. Hide Caption 28 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A police officer checks cars entering the Brooklyn Queens Expressway to confirm that they have three occupants before allowing them to cross into Manhattan on Thursday. Limited public transit has returned to New York, and most major bridges have reopened. However, vehicles must have three occupants to pass. Hide Caption 29 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Parts of lower Manhattan are still without electricity on Thursday. Superstorm Sandy, which made landfall along the New Jersey shore on Monday, October 29, left much of the Eastern Seaboard without power, including much of Manhattan south of 34th Street. Hide Caption 30 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Residents of New York City's East Village enjoy a bonfire on Wednesday, October 31. Hide Caption 31 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – East Village residents charge their phones with power from a generator on Wednesday. Hide Caption 32 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Traffic snarls in New York City on Wednesday. Residents and businesses across the Eastern Seaboard are attempting to return to their daily lives in the aftermath of Superstorm Sandy. Hide Caption 33 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A Caring Foundation worker hands out food to residents of the heavily damaged Rockaway section of Queens on Wednesday. Hide Caption 34 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Water floods streets in the Rockaway section of Queens on Wednesday. Hide Caption 35 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Traders stand outside of the New York Stock Exchange in New York on Wednesday. Stocks advanced as U.S. equity markets resumed trading for the first time this week after the storm. Hide Caption 36 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A man surveys damage on Wednesday, October 31, in the Rockaway neighborhood of Queens, New York, where the historic boardwalk was washed away during Superstorm Sandy. Hide Caption 37 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Residents walk with their belongings through the Rockaway section of Queens on Wednesday. Hide Caption 38 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – The remains of homes burned down in Rockaway are seen Wednesday, a day after an inferno spread across the flooded neighborhood. Hide Caption 39 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Firefighters continued to survey the damage in Rockaway on Wednesday. At least 80 homes were destroyed. Hide Caption 40 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – People wait for buses on Sixth Avenue in New York on Wednesday as New Yorkers cope with the aftermath of Hurricane Sandy. Hide Caption 41 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – People attempt to squish into a crowded bus on First Avenue in New York on Wednesday. Hide Caption 42 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Con Edison crew members work on a steam pipe on First Avenue on Wednesday. Hide Caption 43 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – The foundations to the historic Rockaway boardwalk in Brooklyn are all that remain after it was washed away Wednesday during Hurricane Sandy. Hide Caption 44 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – People walk to work Wednesday on a normally busy street near the New York Stock Exchange. Hide Caption 45 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Traders work on the floor of the New York Stock Exchange on Wednesday after it had been closed for two days. Hide Caption 46 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Commuters arrive in Manhattan by ferry from Jersey City, New Jersey, on its first day back in business after Sandy. Hide Caption 47 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – CNN iReporter Jordan Shapiro captured this view of the Williamsburg Bridge in New York at 11 p.m. on Tuesday, October 30. Half of the bridge and Brooklyn is lit, while the Manhattan side and the surrounding part of the island remain shrouded in darkness. Hide Caption 48 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A subway station and escalator sit underwater in New York on Tuesday. Hide Caption 49 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Much of the New York City skyline sits in darkness Tuesday evening after damage from Superstorm Sandy knocked out power. About 6.9 million customers are without power in 15 states and the District of Columbia, according to figures compiled by CNN from power companies. Hide Caption 50 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – New York City Mayor Michael Bloomberg, center, views the damage Tuesday in the Breezy Point neighborhood of Queens, where a fire broke out during Superstorm Sandy and destroyed at least 80 homes. Hide Caption 51 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A resident looks through the debris of his destroyed home in Breezy Point, Queens, on Tuesday. Hide Caption 52 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Burned-out vehicles and destroyed homes line a street in Breezy Point, located on the western end of the Rockaway peninsula in New York. Hide Caption 53 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A fire continues to burn Tuesday in the remains of a structure that was destroyed by the Breezy Point blaze. Hide Caption 54 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A New York City man hands a dog to first responders while being evacuated on Tuesday. Hide Caption 55 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A bartender at the International Bar in the East Village neighborhood of New York City makes drinks in the dark on Tuesday as electricity remains out for many in the city. Hide Caption 56 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Water floods the Plaza Shops in New York, in the wake of Superstorm Sandy, on Tuesday, October 30. Hide Caption 57 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Con Edison employees monitor the drainage of water being pumped out of Seven World Trade Center in the Financial District of New York on Tuesday. Hide Caption 58 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Onlookers watch a construction crane dangling from a $1.5 billion luxury high-rise after collapsing in high winds. Hide Caption 59 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – The construction crane dangles from a high-rise in midtown Manhattan. Hide Caption 60 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Ramiro Arcos clears debris from a storm drain in the Financial District of New York after Sandy swept through the city. Hide Caption 61 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – A couple walks in the rain Tuesday, with the East River and the Lower Manhattan skyline as a backdrop. Hide Caption 62 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – The Manhattan skyline remains dark after much of the city lost electricity in the storm. Hide Caption 63 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – Cars float in a flooded below-street-level parking area in the Financial District on Tuesday. Hide Caption 64 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – People take a Tuesday morning walk on the Brooklyn Bridge, which remains closed to traffic after the city awakened to the storm damage. Hide Caption 65 of 66 Photos: Photos: New York recovers from Sandy New York recovers from Sandy – . A car sits crushed by a tree in the Financial District on Tuesday. 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Officials had hoped to keep the hospital running, Aviles said, but an investigation Wednesday revealed that the damage had been too extensive.

"The emergency generators were just not enough to keep this hospital operating for the long term," he said.

Bellevue is along the East River, just north of Manhattan's 26th Street.

Floodwaters poured into the massive hospital's 1-million-square-foot basement, Aviles said, damaging fuel pumps and water pumps that supply the complex. None of the hospital's 32 elevators is working, he said.

"It was obviously not anticipated that we would get a storm surge of this magnitude. ... We've never seen anything like this at Bellevue Hospital," Aviles said.

Officials estimate that 17 million gallons of water rushed into the hospital's basement, he said.

Most of the critically ill patients have already been evacuated, said Dr. Sanjay Gupta, CNN's chief medical correspondent, who was reporting from outside the hospital Wednesday. Gupta said moving such patients is a particularly difficult process.

"It can be very challenging ... even to transfer them within the hospital from one floor to the next. That can be a real challenge," Gupta said. "It's a very coordinated process. You always plan for the worst-case scenario. Everything from a patient's heart rate to their body temperature can change."

Gupta said evacuating the remaining patients is "likely to be more methodical, a little bit slower, perhaps even look a little more organized than over the last 24 hours."

Aviles told reporters that it was too soon to tell when the hospital would be up and running again.

"If we can get this hospital back and operating in two or three weeks, we will be doing very well," he said.

Bellevue is just a few blocks away from NYU's Langone Medical Center, where staff evacuated about 260 patients -- including newborn babies that had been in intensive care -- after the storm hit.

At times with only flashlights to illuminate the way, hospital employees carried some patients down 15 flights of stairs to ambulances ready to take them to the safety of other hospitals.

Langone didn't anticipate such heavy flooding from Sandy and chose not to evacuate all its patients before the storm.

But as the storm hit Monday night, 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."