Luminous, usually spherical objects which vary from pea-sized to several metres in diameter. Usually associated with thunderstorms.
Laboratory experiments have produced effects that are visually similar to reports of ball lightning, but it is presently unknown whether these are actually related to any naturally occurring phenomenon. source Wikipedia
Although extensively documented and even photographed, Ball Lightning is currently unexplained.
Quote from Emeritus Professor Bob Crompton of the Australian National University
“I don't believe there is any satisfactory explanation so far”
“[The theories] don't satisfy me and I don't think they satisfy anyone who looks at the evidence objectively.”
source ABC Science , 2008.