Sometime around 4 p.m. yesterday, a power disruption along the U.S.-Canadian border near Lake Erie cut electrical service across the region, blacking out cities spread as far as Canada, New Jersey, Vermont, Ohio and Michigan.
That startlingly rapid chain reaction was caused by interconnections along the same electric grid that usually keeps power flowing smoothly from the Eastern Seaboard to the Great Plains.
The grid - a complex web of interconnected high-voltage lines and power plants - moves electricity from region to region, wherever demand arises.
The high-voltage lines of the grid are connected through transformers to smaller wires that carry power into homes and businesses.
Electricity, unlike other commodities, cannot be stored. It must be moved and used as soon as it is generated.
At the speed of light, electricity will follow the path of least resistance as electrons flow along any available power line.
To allow the grid to operate smoothly, the generation of power and the amount of power consumed by customers, often referred to as "load," must always be in balance.
"The electric system is a very complex thing," said Luther Dow, director of power delivery and markets at the Electric Power Research Institute in Palo Alto, Calif. "There are three separate interconnections, or grids. It's to the benefit of everyone to have those interconnections. ... It's what makes the system work so well when conditions are normal.
"But when we have a major catastrophe like this, the grid also allows for the potential for major outages to occur."
Late yesterday afternoon, something near Niagara Falls - Canadians believed it was a lightning strike; Americans disagreed - threw portions of the grid called the Eastern Interconnection out of balance.
"If you have an overload in one area, that then begins to overload the next area, and then that begins to overload the next and so on," said Bill Lohrman, manager for market interface for the North America Electric Reliability Council, a nonprofit organization whose mission is to ensure that the bulk electric system in North America is reliable, adequate and secure.
"To not overload the system, there are circuit breakers that open up to protect areas when a problem occurs," Lohrman said.
But sometimes breakers or engineers in control centers fail to react quickly enough to avoid widespread disruption.
Energy experts said that when yesterday's disruption occurred, electricity from generation plants in other regions immediately began surging toward the New York area to make up for the loss of energy.
New York's smaller grid is connected to grids in New England, Canada and the mid-Atlantic region.
When the power lines became overloaded as electricity moved toward New York, circuit breakers started automatically opening to create what is known as a "cascading outage."
In other words, the blackouts started when the breakers began disconnecting power plants and entire regional connections from the system to protect them from operating abnormalities.
"I suspect that it was a relatively large plant, if it's true," Dow said. "If you were to lose a major power plant, you'd have a mismatch between the load and the generation, and the system will automatically drop the load.
"To understand, think of your house when you overload a wall socket," Dow said.
"A circuit breaker opens up and disconnects that outlet from the rest of the system to protect the other parts of the system. When New York lost power, the system shut down to protect the rest of the grid."
While such disconnections protect individual plants and pieces of the system from damage that might be caused by power surging out of control, getting the system running again after the trouble has been isolated isn't easy.
That's because the system has to be put back together one small piece at a time to avoid new power surges.
Nuclear power plants that supply much of the region's power were automatically shut down by the outages. They require extraordinary care when they are restarted.
All of that meant people in New York City and across the region face waits for utility engineers to bring the systems back to normal service.
Then, the utilities will face the troubling challenge of determining just why the blackout moved so far so fast.