Mysterious smoke rings that have been appearing around the world may have a mundane explanation, according to UFO expert Nick Pope.

"Despite exotic theories about UFOs, aliens, or portals to other dimensions, I believe there's a decidedly down-to-earth explanation here," said Pope, a former Ministry of Defence employee who investigated UFOs for the British government.

"A so-called smoke vortex can arise when you have a blast through a circular structure like a smokestack."

"So these weird effects can be caused by accident - or indeed by design."

"On this latter point, they can be created as a sort of pyrotechnic effect, and it would be interesting to know if there were any outdoor festivals being held in the area concerned."

The smoke rings have been reported over events such as a military reenactment in England (pictured above), and the music festival Bonnaroo in Tennessee, USA (see the video below). The weird rings appear suddenly and tend to linger in the sky, and can be quite baffling to onlookers who are unaware of their cause.