A new paper says dozens—and by one account about four thousand—US mines exploded during the Vietnam war for a pretty strange reason, Atlas Obscura reports. Published in Space Weather, the paper concludes that a 1972 solar storm triggered the mines: "I was completely taken aback" by the solar event's power, says co-author Delores Knipp. She and her team found declassified US Navy documents that blamed powerful solar activity for the detonations off the coast of North Vietnam near Hai Phong port, per Popular Mechanics. They also dug up little-known scientific reports around the world saying power grids were affected, satellites damaged, and Air Force sensors fooled into turning on as if a nuclear bomb had gone off, per Scientific American.
Four solar flares erupted in early August 1972, the third being a coronal mass ejection—a huge emission of radiation particles and high-energy plasma hurling electromagnetic pulses into space. In a quick 15 hours the pulses slammed into Earth and set off the mines, which were built to detect magnetic fields from passing ships, per a press release. On the upside, it inspired the Navy to make mines more resistant to solar flares. Today it reminds us to guard Earth's infrastructure against such storms, which Knipp says occur about every 70 years. The worst on record was the Carrington Event of 1859: "It's our poster child of storms," says Knipp. "If a storm that bad were to appear again, then we would really have a lot of problems." (A 2012 solar blast nearly sent Earth to the Stone Age.)