The most devastating storm in decades to hit the most densely populated U.S. region cut off modern communication and left millions without power Tuesday, as thousands who fled their waterlogged homes wondered when life would return to normal.

A weakening Sandy, the hurricane turned fearsome superstorm, killed at least 50 people, many hit by falling trees, and still wasn't finished.

It inched inland across Pennsylvania, ready to bank toward western New York state to dump more of its water and likely cause more havoc Tuesday night. Behind it: a dazed, inundated New York City, a drenched Atlantic Coast and a moonscape of disarray and debris — from unmoored shore-town boardwalks to submerged mass-transit systems to delicate presidential politics.

"Nature," said New York City Mayor Michael Bloomberg, assessing the damage to his city, "is an awful lot more powerful than we are."

More than 8.2 million households were without power in 17 states as far west as Michigan. Nearly two million of those were in New York, where large swaths of lower Manhattan lost electricity and entire streets ended up under water — as did seven subway tunnels between Manhattan and Brooklyn at one point, the Metropolitan Transportation Authority said.

"Restoring power and mass transit remain the two biggest challenges in the days ahead," Bloomberg said. "That recovery is a mammoth job."

Some bus routes were up and running, and Bloomberg said an arrangement had been made to try to get extra taxi service on city streets.

The New York Stock Exchange was closed for a second day from weather, the first time that has happened since a blizzard in 1888. The city's subway system, the lifeblood of more than five million residents, was damaged like never before and closed indefinitely, and Consolidated Edison said electricity in and around New York could take a week to restore.

"Everybody knew it was coming. Unfortunately, it was everything they said it was," said Sal Novello, a construction executive who rode out the storm with his wife, Lori, in the Long Island town of Lindenhurst, and ended up with about two metres of water in the basement.

The scope of the storm's damage wasn't known yet. Though early predictions of river flooding in Sandy's inland path were petering out, colder temperatures made snow the main product of Sandy's slow march from the sea. Parts of the West Virginia mountains were blanketed with about 60 centimetres of snow by Tuesday afternoon, and drifts more than a metre deep were reported at Great Smoky Mountains National Park on the Tennessee-North Carolina border in the South.

"We've had crews working around the clock, and we had a pretty rough go of it last night, but since the daylight hours we've been able to get the road in good shape," said Ron Hamilton, maintenance director for West Virginia Parkways.

A parking lot full of yellow cabs is flooded as a result of superstorm Sandy on Tuesday in Hoboken, NJ. (Charles Sykes/Associated Press)

By Tuesday afternoon, there were still only hints of the economic impact of the storm. Airports remained closed across the East Coast and far beyond as tens of thousands of travellers found they couldn't get where they were going.

Forecasting firm IHS Global Insight predicted the storm will end up causing about $20 billion in damages and $10 billion to $30 billion in lost business. Another firm, AIR Worldwide, estimated losses up to $15 billion -- big numbers probably offset by reconstruction and repairs that will contribute to longer-term growth.

Battered beaches

Images from around the storm-affected areas depicted scenes reminiscent of big-budget disaster movies. In Atlantic City, New Jersey, a gaping hole remained where once a stretch of boardwalk sat by the sea.

The governor said Tuesday evening that authorities had confirmed six Sandy-related deaths in New Jersey.

"What you realize is the power of this storm surge," he said Tuesday after surveying the damage. "It was the wind and the storm surge from the ocean that did the damage that it did here."

He said he also noticed the "incredible erosion" on the beach in some areas.

"It was an overwhelming afternoon for me, emotionally," he said Tuesday evening as he described the wreckage, including iconic rides that been swept into the Atlantic.

"We'll rebuild it," he said about the Jersey Shore. "No question in my mind, we'll rebuild it."

But the governor, who was born and raised in New Jersey, said the shore of his youth was gone.

Christie said he expected the losses would be in the billions, but he said there were no precise figures this early into the recovery effort.

In Queens, New York, rubble from a fire that destroyed as many as 100 houses in an evacuated beachfront neighbourhood jutted into the air at ugly angles against a grey sky. In heavily flooded Hoboken, New Jersey, across the Hudson River from Manhattan, dozens of yellow cabs sat parked in rows, submerged in murky water to their windshields. At the ground zero construction site in lower Manhattan, sea water rushed into a gaping hole under harsh floodlights.

One of the most dramatic tales came from lower Manhattan, where a failed backup generator forced New York University's Tisch Hospital to relocate more than 200 patients, including 20 babies from neonatal intensive care. Dozens of ambulances lined up in the rainy night and the tiny patients were gingerly moved out, some attached to battery-powered respirators as gusts of wind blew their blankets.

Sandy killed 18 people in New York City, the mayor said. The dead included two who drowned in a home and one who was in bed when a tree fell on an apartment. A 23-year-old woman died after stepping into a puddle near a live electrical wire.

In a measure of its massive size, in the Midwest waves on southern Lake Michigan rose to a record-tying 6.1 metres. High winds spinning off Sandy's edges clobbered the Cleveland area early Tuesday, uprooting trees, closing schools and flooding major roads along Lake Erie.

The presidential candidates' campaign manoeuvrings Tuesday revealed the delicacy of the need to look presidential in a crisis without appearing to capitalize on a disaster. President Barack Obama cancelled a third straight day of campaigning, scratching events scheduled for Wednesday in Ohio, in Sandy's path. Republican challenger Mitt Romney resumed his campaign with plans for an Ohio rally billed as a "storm relief event."

Dozens of homes were devastated by fire in the Breezy Point section of the Queens borough of New York. (Shannon Stapleton/Reuters)

Sandy began in the Atlantic and knocked around the Caribbean — killing nearly 70 people — and strengthened into a hurricane as it chugged across the southeastern coast of the United States. By Tuesday night it had ebbed in strength but was joining up with another, more wintry storm — an expected confluence of weather systems that earned it nicknames like "superstorm" and, on Halloween eve, "Frankenstorm."

Around midday Tuesday, Sandy was about 193 kilometres east of Pittsburgh, pushing westward with 72 km/h and was expected to turn toward New York State on Tuesday night. Although weakening as it goes, the storm will continue to bring heavy rain and flooding, said Daniel Brown of the National Hurricane Center in Miami.

In Canada, power crews are still working to restore electricity to thousands of people after a night of rain and powerful winds in parts of Ontario, as well as some areas in Quebec and the Maritimes.

Jean-Marc Couturier, of the Canadian Hurricane Centre, said that some areas in the Maritimes are still under rainfall warnings.

"It looks like we'll hold on to these warnings probably through the day tomorrow," he said.

With files from CBC News