Scientists long doubted centuries of witness claims of lights in the sky around the time of an earthquake. But photographic evidence supports the reports, and experts are taking the phenomenon seriously. "Earthquake lights are a real phenomenon—they’re not UFOs," Robert Thériault, the lead author of a new study on the matter, tells Nature. His theory holds that "deep in the crust," rocks form electric charges when pushing against each other; the charges move toward the Earth's surface along geological faults.
When the ions hit the air, they create a charged gas that is the source of the light, the Smithsonian explains. Thériault's team reached its conclusion by gathering data on 65 earthquakes since 1600 that were linked to lights in the sky. The author offers some advice: "If you see visible lights in the sky, and you live in an earthquake-prone area, they might be an early-warning sign that an earthquake is approaching," he notes. Indeed, spotting such lights prompted a man to rush his family outside in L’Aquila, Italy, in 2009—ensuring their safety two hours before that devastating earthquake hit, Nature reports. (Read more earthquake stories.)