After a major earthquake struck off the southern coast of Mexico on Sept. 7, videos of fuzzy green smears in the night sky went viral online.
Earthquake lights are a phenomenon so unusual that they border on myth. The first known reports of them are from 89 B.C., with spotty descriptions over the centuries.
Recently, they've been seen during foreshocks and the main earthquake in L'Aquila, Italy, in 2009, and as flashes of blue lightning over Wellington, New Zealand, at the time of the earthquake there in 2016.
"These phenomena are well documented because of so many security cameras running day and night now," says Friedemann Freund at NASA's Ames Research Center in Mountain View, Calif.
Earthquake lights are electric discharges that come out of the ground and can rise up to more than 200 yards in the air, Freund says. Lights are sometimes observed in the days and hours leading up to an earthquake; in other cases, they are visible during or after a quake. How they work is a mystery.
Part of the challenge in learning more is that earthquake lights have been described inmany ways: as glowing globes, flickering flames from the ground, or even branches of lightning originating from the surface instead of the sky.
In many instances, they could be explained by other phenomena, such as volcanic flames from fissures, streaking meteors and auroras.
"It's a little bit difficult to ferret out what is a real one and what is not," says Troy Shinbrot at Rutgers University in New Jersey. But what's actually happening?
One idea is that when igneous or metamorphic rocks are under stress, their molecular bonds break and release ionized oxygen that travels through the rock. "The faster we stretch the rocks, the more of these positive-charge carriers are released," Freund says.
In this line of thinking, some of these ions can create charged layers at or just below the surface, generating localized electric fields. The strongest fields cause coronal discharges — brief bursts of visible light.
In some conditions, defects in the mineral structure mean that rocks can even behave as semiconductors that explode as a flash of light when hit by seismic waves. This state lasts for only fractions of a second, Freund says, and then charged particles "can break through Earth's surface and discharge as an electric discharge into the air."
Another possibility is that earthquake lights are a manifestation of triboluminescence, which refers to light released when chemical bonds are broken through rubbing, crushing or scratching. This effect has been demonstrated under certain circumstances by tumbling mixed grains in small-scale laboratory experiments.
Similarly, squeezing quartz pushes surface ions out of position, generating a tiny electric current. It's plausible that the same processes could create earthquake lights when faults move and crush rocks together, given that quartz is one of the most common minerals in Earth's crust.
No matter the idea, it would be difficult to test it in the real world. "However you do the experiment, there's going to be an inevitable big gap between how large an experiment can be — a meter, two meters — and tens of kilometers, which occur in earthquakes," Shinbrot says.
But one thing is clear: With more viral footage emerging every few months, scientists will have plenty of data to sift through.