On an April day in 2009, bizarre four-inch flames of light were seen hovering above a stone-paved road in the historical city center of L'Aquila, Italy. Shortly after, a cataclysmic magnitude 6.3 earthquake devastated the area reportedly leaving about 300 people dead.
At the time, these light-filled flashes were thought to be a coincidental phenomenon, but now researchers believe they had a direct correlation to the earthquake.
A new study published in Seismological Research Letters says these flashes of light rarely seen before or during earthquakes are caused by naturally occurring electrical processes in certain types of rock.
L'Aquila was one of several places to see such lights before an earthquake. Other instances include the 1906 earthquake in San Francisco, Calif., where locals witnessed a rainbowed light beam above a street right before the temblor, and the 1988 earthquake in Quebec, Canada, where people saw a purplish glowing sphere near the St. Lawrence River 11 days before the quake, according to National Geographic.
The lights can come in "many different shapes, forms, and colors," study coauthor Friedemann Freund, an adjunct professor of physics at San Jose State University and a senior researcher at NASA's Ames Research Center, told National Geographic. Not only are there globes of light and flickering flames, but some earthquake lights look like quick bursts of lightning coming straight out of the ground.
Past explanations for these strange colorful lights that preceded earthquakes were UFOs, birds, and planes. The phenomena is rare -- it only happens in less than 0.5 percent of earthquakes -- which would explain why some witnesses have claimed they were caused by aliens.
The study's authors say the lights happen because of miniscule defects in the crystals of basalt and gabbro rocks. When these rocks are impacted with seismic activity, they let off electrical charges.
"When nature stresses certain rocks, electric charges are activated, as if you switched on a battery in the Earth's crust," Freund told National Geographic. "The charges can combine and form a kind of plasma-like state, which can travel at very high velocities and burst out at the surface to make electric discharges in the air."
While it seems like these lights would be a good warning system for upcoming earthquakes, Freund said the lights are too rare for predicting shifts in the Earth's tectonic plates. However, paired with other earthquake indicators, they could help with forecasting.
"If we see two, three, or four characteristic phenomena, then it looks like there might be an earthquake," he told National Geographic. While the earthquake lights are rare, he said, "If they are observed, let's watch out."
Updated January 7 at 10:35 a.m. PT: removed references to the lights seen before the 2008 earthquake in Sichuan, China, because those lights may have been infralateral or circumhorizontal arcs rather than earthquake lights.