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PITTSBURGH » The most devastating storm in decades to hit the country’s most densely populated region upended man and nature as it rolled back the clock on 21st-century lives, cutting off modern communication and leaving millions without power Tuesday as thousands who fled their water-menaced homes wondered when — if — 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 to dump more of its water and likely cause more havoc Tuesday night. It left behind a dazed, inundated New York City, a waterlogged Atlantic Coast and a moonscape of disarray and debris — from unmoored 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 2 million of those were in New York, where large swaths of Lower Manhattan lost electricity and entire streets ended up underwater — as did seven subway tunnels between Manhattan and Brooklyn at one point, the Metropolitan Transportation Authority said.

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 shutdown of mass transit crippled a city where more than 8.3 million bus, subway and local rail trips are taken each day, and 800,000 vehicles cross bridges run by the transit agency.

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 7 feet of water in the basement.

The scope of the storm’s damage wasn’t completely known. Though early predictions of river flooding in Sandy’s inland path were petering out, colder temperatures made snow the main product of the storm’s slow march from the sea. Parts of the West Virginia mountains were blanketed with 2 feet of snow by Tuesday afternoon, and drifts 4 feet deep were reported at Great Smoky Mountains National Park on the Tennessee-North Caro­lina border.

As organized civilization came roaring back Tuesday in the form of emergency response, recharged cellphones and the reassurance of daylight, harrowing stories emerged from Maryland north to Rhode Island.

Images from around the storm-affected areas depicted scenes reminiscent of big-budget disaster movies. In Atlantic City, N.J., a gaping hole remained where once a stretch of boardwalk sat by the sea. In Queens, N.Y., rubble from a fire that destroyed as many as 100 houses in an evacuated beachfront neighborhood jutted into the air at ugly angles against a gray sky. In heavily flooded Hoboken, N.J., across the Hudson River from Manhattan, dozens of yellow cabs sat parked in rows, submerged in murky water to their windshields.

One of the most dramatic tales came from Lower Manhattan, where a failed backup generator forced New York University’s Langone Medical Center 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.

Economic forecasting firm IHS Global Insight predicted the storm will end up causing about $20 billion in damage 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 economic growth.

"The biggest problem is not the first few days, but the coming months," said Alan Rubin, an expert in nature disaster recovery.

The storm grounded more than 18,000 flights across the Northeast and the globe, and travel will take days to return to normal.

According to the flight-tracking service FlightAware, more than 7,000 flights were canceled on Tuesday alone.