At least 6 million people across the mid-Atlantic region are without power Tuesday morning after Hurricane Sandy inundated homes and underground infrastructure, toppled trees and downed power lines. Surreal scenes of a darkened Manhattan emerged after flooding and a substation explosion Monday night knocked out power for most buildings below 39th Street.

A statement on New York utility Con Edison’s website said more than 650,000 customers lost power in “the largest storm-related outage in our history.” The utility predicted that many outages, particularly those related to the substation explosion at East 14th Street, would last up to a week. (See related “Photos: World’s Worst Power Outages“)

Also Monday night, New Jersey’s Oyster Creek Generating Station, the oldest nuclear plant in the U.S., declared an alert due to high water levels, and three reactors experienced shutdowns. A unit at Indian Point in Buchanan, New York, automatically shut down “as a result of electrical grid disturbance,” according to plant operator Entergy. The other shutdowns occurred at Nine Mile Point in Scriba, New York, when an electrical fault triggered an automatic shutdown; and Salem Generating Station in Hancocks Bridge, New Jersey, which was was manually shut down because of high river levels and debris in the waterway of its circulating-water pumps, according to the U.S. Nuclear Regulatory Commission (NRC).

Oyster Creek’s operator, Exelon, said in a statement Tuesday that water levels “are falling rapidly today as Hurricane Sandy moves out of the area. When water levels fall sufficiently the station will terminate the Alert.” (An Alert is the second-lowest of four action levels defined by the NRC.) The NRC said in a statement that it was maintaining a “heightened watch” over all affected plants.

Early reports suggested that East coast refineries, which had shut down in anticipation of the storm, were not seeing any major damage after the storm. Philadelphia Energy Solutions, the Northeast’s largest refinery, was reportedly restarting some operations already Tuesday morning.