A “megaflash” of lightning recorded in the Great Plains of the United States has been recognized as the longest ever, stretching 829 km (about 515 miles) across five states, from eastern Texas to Missouri.

These megaflashes—extended lightning discharges—are associated with large clusters of thunderstorms typically found in the Great Plains and other lightning-prone areas. The reasons why some storms produce megaflashes while others do not remain a mystery. “I don’t know exactly why this happens,” says Michael Peterson from the Georgia Institute of Technology.

The lightning event occurred in October 2017 and lasted for over seven seconds. At the time, it was too lengthy to be completely mapped by ground-based lightning detection systems.

To verify the extent of this flash, Peterson and his team returned to satellite data from Earth-based cameras that continuously monitor lightning. They utilized new software to process millions of light pulses, successfully tracking the lightning as it traveled through the storm.

“Every new pulse gives you an idea of how that flash evolves over time,” Peterson explains. “It’s like playing Connect-the-Dots to piece together the entire structure.”

They discovered the flash’s maximum linear length of 829 km, plus or minus 8 km—comparable to the distance between London and Zurich. However, the total trace along the bolt itself exceeds 1000 kilometers. “The Mega Flash can extend to the Megameter,” Peterson adds.

The World Meteorological Organization, which maintains records of extreme weather events, has confirmed this flash as the longest ever documented, surpassing prior records by approximately 61 kilometers since 2020.

“From this single flash, we observed over 100 cloud-to-cloud strokes,” Peterson notes. “Although mega flashes are rare, each one is quite remarkable.”