Canberra, Jun 25 (IANS): An Australian entrepreneur is offering a cash reward of A$5,000 ($3,710) to decipher one of the country's greatest unsolved modern mysteries, a media report said on Monday.

On a desert plateau 36 miles from the tiny South Australian town of Marree, lines run through the barren earth, carved one foot deep and over 115 feet wide, reports CNN.

Stretching over 2.6 miles, the lines join together to form an image of an Aboriginal figure wielding a stick or boomerang.

This geoglyph, one of the world's largest, is known as the Marree Man.

In the 20 years since it first appeared in 1998, nobody has stepped forward to claim responsibility, giving rise to countless theories and investigations.

Dick Smith, founder of Dick Smith Electronics and Dick Smith Foods, decided to tackle the mystery in 2016.

In the last two years, he and his team have pored over images and videos, investigated the various theories and reviewed the few pieces of evidence available but to no avail.

He believes a group was responsible and is now hoping that a sizable cash award will help bring forward new information.

"There were no mistakes... It was very professionally done," Smith told the Australian Broadcasting Corp News 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?"

The most popular theory is that an American or a group of Americans were behind the geoglyph, CNN reported.

Immediately after it was discovered, a series of press releases were anonymously faxed to the Marree Hotel and to The Advertiser daily, describing "the world's largest work of art".

An alternative theory credits Aboriginal artist Bardius Goldberg with the work.

Goldberg, who lived in Alice Springs, had apparently told friends he was responsible for the Marree Man. However, this was never confirmed and Goldberg died in 2002.