On August 14, 2003, a software bug in the alarm system at FirstEnergy's control room blinded operators to a voltage drop, causing a cascading failure that plunged 55 million people across the U.S. Northeast, Midwest, and Ontario, Canada, into darkness—making it the second-widespread blackout in world history at the time. The event began with a 3,500 megawatt power surge toward Ontario, which rapidly overwhelmed the grid, shutting down 508 generating units at 265 power plants and causing outages in major cities like New York City, Toronto, Detroit, and Cleveland.

The immediate chaos was immense: airports were closed, Amtrak and commuter railways halted, and cell service was overloaded, while water systems in several cities lost pressure. High heat—exceeding 31°C (88°F)—was a key factor, as it increased demand and caused power lines to sag, contributing to the initial fault. Although most areas restored power within 7 hours, New York City and parts of Toronto did not fully recover until August 16, with the subway resuming limited service just hours after the blackout began. This historic failure highlighted the vulnerability of the electrical grid, where a single software error combined with environmental stress can trigger a massive, multi-state outage.