Rare flashes of light that are sometimes seen around earthquakes are not  caused by birds, or planes, or UFOs—all of which had been previously used to explain the phenomena known as earthquake lights.

Instead, the lights are caused by electrical properties of certain rocks in specific settings, report scientists in a new paper.

Sometimes called earthquake lightning, the lights can take "many different shapes, forms, and colors," says study coauthor Friedemann Freund, an adjunct professor of physics at San Jose State University and a senior researcher at NASA's Ames Research Center.

Freund says common forms of earthquake lights include bluish flames that appear to come out of the ground at ankle height; orbs of light called ball lightning that