<p><strong>A full moon created an eerie skyline silhouette after New York City went dark during the blackout of November 1965. In a world that's increasingly dependent on constant power, massive electrical outages are a common concern and may strike systems across the globe.</strong></p><p>Major power disturbances can be triggered by storms, heat waves, solar flares, and many other sources, but all have roots in the mechanical and human vulnerabilities of the power grids themselves. "Power delivery systems have a lot of parts, wires, transformers, and other components all nicely tied together—which means there are a lot of things that can go wrong," explained Clark Gellings of the nonprofit <a href="http://my.epri.com/portal/server.pt">Electric Power Research Institute</a>. "Pieces break down and people make errors. A system is designed to tolerate a certain amount of disruption but past a certain point, it's simply gone too far and it falls apart."</p><p>The "great Northeast blackout," which began when a power surge near Ontario set off a chain of failures across New York State and beyond, covered 80,000 square miles. "Within four minutes the line of darkness had plunged across Massachusetts all the way to Boston," reported <em><a href="http://www.nytimes.com/learning/general/onthisday/big/1109.html">The New York Times</a></em> on the day of the outage. "It was like a pattern of falling dominoes—darkness sped southward through Connecticut, northward into Vermont, New Hampshire, Maine, and Canada."</p><p>—<em>Brian Handwerk</em></p>

Northeastern U.S. and Canada, 1965

A full moon created an eerie skyline silhouette after New York City went dark during the blackout of November 1965. In a world that's increasingly dependent on constant power, massive electrical outages are a common concern and may strike systems across the globe.

Major power disturbances can be triggered by storms, heat waves, solar flares, and many other sources, but all have roots in the mechanical and human vulnerabilities of the power grids themselves. "Power delivery systems have a lot of parts, wires, transformers, and other components all nicely tied together—which means there are a lot of things that can go wrong," explained Clark Gellings of the nonprofit Electric Power Research Institute. "Pieces break down and people make errors. A system is designed to tolerate a certain amount of disruption but past a certain point, it's simply gone too far and it falls apart."

The "great Northeast blackout," which began when a power surge near Ontario set off a chain of failures across New York State and beyond, covered 80,000 square miles. "Within four minutes the line of darkness had plunged across Massachusetts all the way to Boston," reported The New York Times on the day of the outage. "It was like a pattern of falling dominoes—darkness sped southward through Connecticut, northward into Vermont, New Hampshire, Maine, and Canada."

Brian Handwerk

Photograph by Bob Gomel, Time Life Pictures/Getty Images

Pictures: World’s Worst Power Outages

From weather events to human error, a range of snafus can wreak havoc for millions when they create the massive blackouts that have periodically struck regions around the world.

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