Entrepreneur Dick Smith has offered a $5000 reward for any information to help identify the person behind the renowned Marree Man desert artwork.
The geoglyph, located in an arid stretch of South Australia, appeared in 1998, but its creator has never been found.
At more than three kilometres long, it appears to depict a human figure holding a stick upwards.
Mr Smith has spent two years investigating the mystery without any significant leads.
"There's been so many different claims and the only one I don't believe in is that it was done from outer space," Mr Smith told ABC Radio Adelaide on Monday.
How has it been kept secret for 20 years? Dick Smith
"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?" he said.
"There were no mistakes, it was very professionally done."
Dick Smith Source: AAP
American made?
One popular theory is that a group of Americans are behind the geoglyph.
At the time of its creation, a series of anonymous faxes alerted media to the site, which included American spelling and terminology.
The faxes gloated that it was "the world's largest work of art".
And a plaque featuring an American flag was also reportedly found near the site.
Another theory is that it was made by late Alice Springs artist Bardius Goldberg, who was said to be interested in creating an artwork visible from space.
Yet another theory is that it was created by members of the Australian army.
Mr Smith also told ABC Radio Adelaide he would be willing the keep the secret to himself.
"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."
Mysterious outback SA icon Marree Man. Source: AAP
"But then I'm going to tell everyone: 'I know who did it, but unfortunately they don't want to be known so you're going to have to do your own work to work it out'."
Additional reporting: AAP
