(Map: Jim McMahon)

Monday, August 18—The lights went out in New York City, Detroit, Cleveland, Toronto, and nearly everywhere in between at 4:11 p.m. on Thursday. Tens of millions of people lost power in the biggest blackout in U.S. history.

The blackout was caused by a chain reaction, beginning with the failure of three power lines near Cleveland, Ohio. A computer system should have warned other power plants that they would have to shoulder a bigger load—but the system was broken. The interconnecting power systems of several U.S. states and Canada became unstable, and then Cleveland shut down.

"It was like taking a light switch and turning it off," said Jim Majer, commissioner of Cleveland Public Power. "It was like a heart attack. It went straight down from 300 megawatts to zero."

More than 100 power plants automatically shut down to protect themselves from the wild power surges. The whole process "essentially took nine seconds," said stunned Michehl Gent, chief of the North American Electric Reliability Council, which keeps an eye on the nation's power. "It happened very quickly."

Airports and tunnels were closed, stranding travelers; subways and elevators came to a halt, temporarily trapping riders. Hospitals and emergency-care centers ran on limited power provided by their own backup generators. And people waited for the power to come back.

Making the Most of It

Many residents of New York City saw an opportunity for a kind of August snow day. Many apartment-dwellers, sweltering from the heat and lack of air conditioning, took to sidewalks and stoops with candles and flashlights. In Tompkins Park of Manhattan's East Village, fire spinners twirled flaming batons and danced to bongo drums. One avenue away, a chef grilled burgers and hot dogs and shrimp free for grateful pedestrians.

"Look—stars!" exclaimed one longtime Manhattanite, pointing up beyond the apartment buildings at an unfamiliar sight.

The Power Grid

Electricity flows through a "power grid," a huge web of stations that shift power from area to area, depending on how much electricity a place needs. Nearly every wall plug in every building connects to the grid.

Politicians demanded an investigation into the causes of the blackout, and some urged President Bush to spend more money beefing up the nation's power grid. "We absolutely need to invest in the grid," said Michigan Governor Jennifer Granholm. Parts of Michigan went dark, and much of the state was without water.

Power trickled back into the darkened areas over the weekend.