It blasted the ocean itself over dunes, seawalls and berms and into downtowns, tunnels and subways. It killed dozens of people, destroyed famed landmarks and amusement parks, pushed houses off their foundations and toppled trees. It virtually shut down New York City, the nation's largest city, with major airports, highways, and bridges and tunnels in and out of Manhattan shut down, just as they were after 9/11.
For millions of people in New York City and elsewhere, the lights remain out, communications remain down and floodwaters, downed trees and power lines still make roads impassable.
However, some of the hardest-hit areas on the East Coast were beginning to take the first steps towards recovery. For instance, some New York bridges, tunnels, highways and airports reopened or were slated to be reopened by Wednesday morning.
So far, Sandy has been blamed for the deaths of at least 50 people, according to The Associated Press, and left more than 8 million customers without power. The number of dead continued to rise by the hour a day after the storm made landfall near Atlantic City, N.J., and rocked states including New Jersey, New York, Pennsylvania, Connecticut, Maryland, North Carolina and West Virginia.
"I just never thought I would see what I saw today -- ever," New Jersey Gov. Chris Christie said. "It won't be same. It will be different because many of the iconic things that made it what it was are not gone and washed into the ocean."
The power outages were spread over 17 states, from Virginia to Maine, and while the number of customers affected topped 8 million, the number of people living without power would be several times that number. The number of power outages topped 2 million customers in New Jersey and half a million in New York City, and approached another million on New York's Long Island.
President Obama issued disaster declarations for New York and New Jersey so that federal aid will be offered to the affected areas to help supplement state and local clean-up efforts.
During a visit to the Red Cross headquarters in Washington, D.C., this afternoon, Obama sent a very clear message to federal agencies.
"Do not figure out why we can't do something. I want you to figure out how we do something," the president said. "I want you to cut through red tape. I want you to cut through bureaucracy. There's no excuse for inaction at this point. I want every agency to lean forward and to make sure that we are getting the resources where they need -- where they're needed as quickly as possible."
The president said mayors and governors who run into any trouble can call him directly at the White House. He praised the heroic efforts of rescuers and helpful community members, but emphasized that recovery is going to take some time.
"It is not going to be easy for a lot of these communities to recover swiftly, and so it is going to be important that we sustain that spirit of resilience, that we continue to be good neighbors for the duration until everybody is back on their feet," Obama said.
Among the hardest hit were New Jersey and New York, where public transportation was shut down, millions lost power and storm surges swamped cars, homes, businesses and boardwalks.
But in the wake of the devastation, states are beginning to make moves toward comebacks.
In New York, the New York Stock Exchange is scheduled to reopen on Wednesday after being closed for an unprecedented two days. New York Mayor Michael Bloomberg was scheduled to ring the opening bell.
The Port Authority of New York and New Jersey, which runs the area's airports and bridges, said it plans to have two major airports -- John F. Kennedy International Airport in Queens, N.Y., and Newark Liberty Airport in New Jersey -- open Wednesday morning at 7 a.m. with limited service. However, LaGuardia Airport in New York City will remain closed amid flooding on the tarmac and other damage.
Public transportation in the city also screeched to a halt as the subway system, rail yards and bus depots were flooded in what officials called the biggest disaster of its 108 years in existence.
"The New York City subway system ... has never faced a disaster as devastating as what we experienced last night," MTA Chairman Joe Lhota said in a statement.
All bridges into Manhattan were reopened today and limited bus service was to resume this evening -- though the Brooklyn Battery Tunnel remained submerged beneath floods. The Holland Tunnel also remained closed though the Lincoln Tunnel was reopened early Tuesday.
Officials hoped to have power restored to New York in two to three days and aim to have the subways running in three to four days, Bloomberg said.
It will take about a week for PATH trains between New Jersey and New York to resume service.
"This was a devastating storm, maybe the worst that we have ever experienced," Bloomberg said at a news conference today.
Obama will visit ravaged New Jersey on Wednesday, where search and rescue missions have become a priority.
A berm in Bergen County, N.J., was breached this morning, resulting in four to five feet of water flowing into three towns and endangering as many as 2,000 people, said Jeanne Beratta, spokeswoman for the Bergen County Office of Emergency Management.
"We're doing rescues by boat. We're doing rescues with large trucks. We're doing rescues all over those areas," Baratta told "Good Morning America." "It's going to continue all day because now we're just search and rescue."
New Jersey Gov. Chris Christie said that the state "kind of took it in the neck worse than any other place," but praised Obama and his administration for how it has handled the crisis.
"[Obama] called me last night around midnight to ask what else can be done," Christie told "GMA." "I have to say, the administration, the president himself and FEMA administrator Craig Fugate have been outstanding with us so far. We have a great partnership with them and I want to thank the president personally for his personal attention to this."
Other parts of the country were struggling with snow and blizzard conditions. West Virginia was under a blizzard warning and more than two feet of snow was reported in some parts of the states. More than 100,000 customers are without power.
Sandy also brought winter conditions from North Carolina to Pennsylvania, and into Ohio.
The former hurricane had joined forces with a cold front coming from the northwest and a high pressure system from Greenland to dump snow on eight states. Davis, W.Va., has been blanketed with 17 inches of snow, which continued to fall into the early morning.
By Thursday, meteorologists predict up to three feet of snow was possible in higher elevations.
New York University Medical Center was among the millions left without power in the wake of Sandy. A full evacuation was under way after the hospital's back-up generators had failed.
Early this morning, approximately 200 patients had been evacuated by private ambulance with assistance from the FDNY.
John Miksad, senior vice president for electric operations at Con Edison, said it was too soon to say when power could be restored and that inspectors would be out once it was daylight to assess the damage.
Sandy was downgraded from a hurricane to a post-tropical storm shortly before it made landfall at 8 p.m. in Atlantic City, N.J., on Monday.
By late Tuesday evening, Sandy remained a tropical cyclone with maximum sustained winds of 40 mph. Though it was weakening over Pennsylvania, it churned up the waters of the Great Lakes, prompting gale warnings and small craft advisories in some locations, according to the National Weather Service. Parts of the mid-Atlantic and Northeast states remained under flash flood watches and warnings.
"Sandy is expected to turn north across Western New York or Lake Erie ... and continue to move northward into Canada on Wednesday," the National Weather Service said in it's 11 p.m. ET briefing Tuesday.
ABC News' Max Golembo, Richard Esposito, Russell Goldman, Michael S. James, Genevieve Shaw Brown, Molly Hunter and Serena Marshall contributed to this report.