They're unexplainable, unpredictable, and rare. Recorded sightings of these hovering, glowing spheres, which can range in size from a penny to several yards across, date back to ancient Greece, but scientists still don't know what causes them. Until recently many people didn't even think they were real.
The most plausible idea to explain ball lightning is vaporized silicon. In this hypothesis, a bolt of lightning strikes a spot of silica-rich soil, sending an orb of charged particles airborne. The inherent electricity causes the orb to glow.
In July 2012 researchers in China inadvertently captured video of this phenomenon, recording a 16-foot-wide ball of lightning on China's Qinghai Plateau. Their findings—released in January 2014—support the vaporized-silicon hypothesis, since there was silicon in the area's soil.