Ball lightning may be all in the mind, according to scientists at the University of Innsbruck.
They've discovered that the magnetic field of long lightning strokes can produce hallucinations of luminous shapes, also known as phosphenes, in the brain.
Many people claim to have seen ball lightning - spheres of glowing light - during thunderstorms. But scientists have struggled to find an explanation.
Josef Peer and Alexander Kendl studied the electromagnetic fields of different types of lightning strokes occurring during thunderstorms.
They found that the magnetic fields of a particular type of long-lasting repetitive lightning discharge show the same properties as transcranial magnetic stimulation (TMS) - used in clinical practice to stimulate neural activity in the brain.
"In the clinical application of TMS, luminous and apparently real visual perceptions in varying shapes and colors within the visual field of the patients and test persons are reported and well examined," says Alexander Kendl.
The findings may not entirely explain ball lightning - scientists believe that the phrase describes more than one type of phenomenon.
Other researchers have produced luminous fire balls in the laboratory. These were a little like descriptions of ball lightning and could explain some observations, but were mostly too short-lived.
Other plausible explanations for some sightings are St Elmo's fire, luminous dust balls or small molten balls of metal.
Kendl’s hypothesis that in fact the majority of ball lightning observations are phosphenes is strongly supported by its simplicity: "Contrary to other theories describing floating fire balls, no new and other suppositions are necessary," he says.
Their findings are published in Physics Letters A.