The massive 8.2-magnitude earthquake that rocked Mexico earlier this week was enough of a phenomenon on its own, but the quake also had something of a fascinating side effect.

The mysterious lights in the sky following the massive earthquake were a shock to those in the area. Numerous videos have been cropping up on social media showing the flashes brighten the night sky above Mexico City, but the flashes aren’t lightning coming from the clouds above, or even lights from planes.

The mysterious lights didn’t immediately have an explanation.  But now, the flashes of light are instead thought to have been caused by the 8.2 magnitude earthquake itself. More specifically, the lights come from a particular type of rock.  A study published in 2014 revealed that these lights can come in many forms, such as bluish flames, orbs of light, or quick flashes that resemble lightning.

“In certain types of rocks this accumulation of stress can break up pairs of negatively charged oxygen atoms in the ground, allowing them to flow up to the surface as an electrical current through cracks in the rock,” said Leila Ertolahti, an adjunct professor of geology at Farleigh Dickinson University, to Gizmodo. “If enough atoms are present they can ionize pockets of air and form a plasma, or charged gas, that emits light.”

As amazing as the appearance of the lights may be, the fact that the earthquake itself occurred was even more historically important.  At an 8.2 magnitude, tremors could be felt from over 966 km (600 miles) from the epicenter, and the number of reported casualties is still climbing. “The scariest part of it all is that if you are an adult, and you’ve lived in this city your adult life, you remember [the 1985 earthquake] very vividly,” said Alberto Briseño, a 58-year-old bar manager in Condesa, to The New York Times. “This felt as strong and as bad, but from what I see, we’ve been spared from major tragedy.”

 The lights may have been terrifyingly beautiful, and the scientific explanation comforting, yet to those affected by this massive earthquake, the apprehension at mother nature’s abilities will always be right below the surface of their emotional psyche.