Here’s what you’ll learn when you read this story:

• One tree instigated the largest blackout in North American history, but the inciting event was the result of poor transmission line management and a perfect storm of technological and training failures.
• Summer heat coupled with increased electricity demand and computer bugs caused several power lines to sag and make contact with trees, causing too much power to flow to other lines and tripping them.
• While power for some was restored after a couple hours, it took other places up to four days, and its estimated that the blackouts led to at least 90 deaths.

The tree of heaven is divine in name only. First brought to the U.S from its native China in the late 1700s, Ailanthus altissima was initially beloved for its absence of insects, disease, and incredible growth—capable of growing eight feet in just its first year. But like many ornamentals brought to the New World with the best of intentions, the tree of heaven soon transformed into a horticultural devil, outcompeting local flora, releasing toxins that kill nearby plants, and even providing habitats for other troublesome animal invaders (including the pernicious spotted lanternfly).

But on August 14, 2003, the tree of heaven upped the ante on its past devilish behavior, causing the largest blackout in the history of North America.

To be fair, it wasn’t entirely the tree’s doing. But at around 2:02 p.m. EST on that hot summer day outside of Cleveland, a sagging 345 kilovolt (kV) transmission line came into contact with an untrimmed tree of heaven, causing a fault. This would be the first in a series of cascading events that led to 256 power plants going completely offline in the northeastern U.S. and Ontario, Canada, impacting roughly 55 million people in major cities like New York, Detroit, Cleveland, New Jersey, and Toronto. For some, the blackout lasted only two hours. For others, it stretched on for four days, disrupting transportation, cellular communication, and even medical care. It’s estimated that the blackout resulted in the deaths of at least 90 people.

Sagging power lines are common, especially in the hot days of summer when increased electrical demand (mostly for air conditioners) and high temperatures cause thermal expansion. Utility companies design transmission lines with a certain amount of sag built in, and create plentiful clearance to prevent electricity from flowing into the ground—at least, in theory.

“There were three T’s that principally contributed [to the 2003 blackout],” Charles Dickerson, president and CEO of the Northeast Power Coordinating Council, said in a video produced by the North American Electric Reliability Corporation (NERC). “One T meaning tree trimming and how vegetation was managed.”

While it’s easy to simply blame the tree of heaven for this catastrophe, the real culprit lies in Dickerson’s next two T’s: tools and training. When that 345 kV transmission line touched A. altissima, the power grid operated by FirstEnergy near Cleveland, Ohio, was already under immense strain. The Midwest Independent System Operator’s (MISO) state estimator—an algorithm used by operators to understand the internal state of a complex system—had been mistakenly shut down, and a vital unit at a power plant northeast of Cleveland tripped offline roughly 30 minutes before the transmission line’s initial woody run-in.

Unfortunately, alarm systems failed to alert operators at FirstEnergy, after which more power lines sagged into trees causing more outages—specifically along the Harding-Chamberlin line, the Hanna-Juniper interconnection, and the Star-South Canton line. Because of the faults with the state estimator and alarms, the operators had no idea that there even were any problems until it was too late. Excess power moved to other lines, which eventually exceeded their operating maximums and tripped off as well, assuming that a fault had occurred. Eventually, the mayhem spread throughout the northeast, and only avoided New England because operators separated from New York to avoid the blackout.

The second largest blackout in the world at the time—and the largest in North America to this day—forced the U.S. government to rethink how it managed the electrical grid. The Natural Resources of Canada and the U.S. Department of Energy eventually released a 240-page report detailing the cause of the widespread 2003 blackout, eventually leading to the Energy Policy Act of 2005, which set federal reliability standards for the grid.

While the grid is an immensely complicated (yet inspiring) feat of engineering, the 2003 blackout reminded us—citizens and politicians alike—that it’s also remarkably fragile. With warming summers and ever-present threats of attacks, calls for hardening the grid and transitioning to clean forms of energy are louder than ever.