People near Marrakesh, Morocco, shared videos recorded before the 6.8-magnitude earthquake that hit the area last Friday, showing unidentified lights in the sky. The bizarre aerial phenomena are known as earthquake lights, and seem to occur above sites of seismic stress.

“People have wondered about them forever,” said Karen Daniels, a physicist at North Carolina State University. “It’s one of those persistent mysteries that hang around and never quite get nailed.”

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