Sixteen years ago, a large part of the United States went dark… for days. August 14 is the anniversary of when parts of Michigan, Ohio, Pennsylvania, New York, New Jersey, Rhode Island, Connecticut, and Massachusetts went without power. It’s estimated that 50-million people were affected, which included parts of Canada.

The blackout had a wide impact in the U.S. including shutting down 21 power plants, disrupting cell phone service, and affecting operations at hospitals, airports, and local governments.

Power for some was restored as early as two hours after the blackout struck, but for most power stayed off for at least one day. Some areas didn’t’ have power for two full weeks.

Immediately following the blackout, thoughts went to terrorism, which it turned out not to be. Trying to figure out the cause, speculation went to a solar flare or a coronal mass ejection (CME). CME’s can cause magnetic storms affecting communication systems, power grids, and astronauts in space.

However, after an investigation, it was revealed in 2004 that the cause of the Northeast blackout of 2003 was because of a number of failures, starting with trees hitting powerlines. The U.S. and Canada put together a task force to investigate the cause of the outage. According to their findings, high voltage powerlines came in contact with overgrown trees. This trip caused a cascading effect with a number of failures along the way. It was determined that FirstEnergy, the company based in Akron, Ohio, was at fault. A software bug within the companies system made operators unaware of the need to redistribute the load after transmission lines dropped out. The report also said that FirstEnergy did not recognize or understand the deteriorating condition of its system and that it failed to manage tree growth in its “transmission rights of way."

Officials say this blackout should have been managed to a local region but instead cascaded to collapse power for a large region.