An Australian physicist says he's solved the mystery of a series of UFO sightings over South-East Queensland in May 2006.
Dr Stephen Hughes of the Queensland University of Technology (QUT) says the green fireballs were caused by meteor fragments accompanied by extremely rare ball lightning.
Hughes examined more than 200 mobile phone photos and reports of the 'UFOs'.
"It was also apparent that at least three fireballs had blazed across the sky at about the same time - one travelling north up the coast, another roughly east to west across Brisbane and another south over the Gold Coast," he says.
"Fireballs can produce ionised trails which, in principle, are able to conduct electricity between 80km to 20km from the Earth. It is possible that they could generate a conductive trail all the way to the Earth that would be invisible till it hit the ground where it would produce a ball-like shape as the current flowed back from the ground."
The fireballs were probably fragments of a comet that passed close to Earth on May 16 2006, he says, glowing green because of oxygen atoms being ionised by the swift passage of the meteor. The same explanation could account for many previous UFO sightings, he says.
"However, one of the most interesting reports was of a 'strange, green, glowing ball' about 30cm in diameter that was seen gently rolling down the slopes of the Great Dividing Range soon after the passage of a green fireball, which was reported by Doug Vernon of Greenmount in Queensland," says Hughes.
This, he reckons, was the rare phenomenon of ball lightning.
"Ball lighting is an elusive event but many reports of it have accumulated over the years and there have been just as many suggestion as to its cause - from mini tornadoes of burning silicon dust to electric knots and mini black holes," he says.
"I think the ball lightning phenomenon Mr Vernon saw was caused by a transient electrical connection between the ionosphere - a layer of electrified air 80km to 600km above Earth - and the ground. "
Dr Hughes' research paper and supplementary material are here.