First-hand experience of UFOs has eluded him, and consequently he maintains a healthy scepticism on the possibility of their existence.

"When I meet someone who says they've seen something strange, that's fair enough, because maybe they have. I don't know what it is, though," he said.

They might include Melburnians who claimed to have been kidnapped by Martians. "They seem like something has happened to them.

"The first time you meet an abductee, as they are called, it can be quite confronting because they are trying to come to terms with it. They don't know what it is either, and it is something that can stay with people a long time."

In Melbourne 15 years ago, Kelly Cahill claimed to have flashbacks of being taken away in a UFO by strange creatures at Narre Warren.

Mr Plowman's research into UFOs took him to Roswell, New Mexico, where it was claimed a flying saucer crashed in 1947, and Wiltshire, England, where crop circles have been attributed to space ships.

The trail led him to Mexico and Latin America, the latest source of reported sightings.

"In the Andes in Chile, they showed me a rock, where they claimed an alien had stepped and left a footprint in the rock."

His interest in UFOs stemmed from boredom as a child while in hospital. His parents gave him a book on UFOs to distract him. "It captured my imagination."

His interest waned and he went on to study physics. But while looking for a topic for his PhD, his interest was reignited when visiting another student's house, where he noticed a bookcase full of books on UFOs.

"Having a look at it, I realised this was a whole world unto itself," he said. "It had rules, and ideas and history and it hadn't been looked at much so my enthusiasm came back and I thought this is a story that I want to look at.