In 1998 a charter pilot flying across outback South Australia noticed an enormous carving on the desert floor, about 60 kilometres west of Marree. The carving seemed to be of a man with a bird’s head holding a spear. It looked like a kind of ancient geoglyph as seen in Europe or South America, but it turned out to be new—apparently graded into the soil with earth moving equipment only months earlier.

Two months later, several anonymous press releases were received by local businesses and newspapers throughout Adelaide. All gloated over the artwork’s creation but none mentioned an author, creating a mystery that’s remained ever since.

And now, 20 years later, one of Australia's richest men—electronics hobby store and foodstuffs tycoon, Dick Smith—is stumping up $5,000 for anyone with answers.

"There's been so many different claims and the only one I don't believe in is that it was done from outer space," Dick told ABC Radio Adelaide on Monday. "I can't see how it was done by one person, you'd have to have three or four to do it, and it would take weeks to put in. In that case, how has it been kept secret for 20 years?"

Dick Smith, who has spent the past two years trying to crack the mystery, then pledged $5K in exchange for a name. He even promised that he wouldn’t tell anyone else, and it’d just be his little secret.

"If somebody rang me up and said, 'Dick, I want the $5000 but on the understanding you won't tell anyone', and then gave me the evidence—I'll give them the $5000 and then keep it secret," he said.

For the moment no one has come forward, and the most compelling theory seems to be that the carving came from a US-based visitor. The first bit of evidence came from the press releases listing the Marree Man’s location in miles, instead of kilometres. Then there was a jar found in late 1998 containing a small US flag and a note that referenced the Branch Davidians, which were the cult at the centre of the 1993 siege in Waco, Texas.

Finally, another plaque was found buried near the Marree Man’s nose in 1999, again featuring an American flag decorated with Olympic rings. It also included a quote from The Red Centre by Australian naturalist H. H. Finlayson. The quote read: “In honour of the land they once knew. His attainments in these pursuits are extraordinary; a constant source of wonderment and admiration.”