This week marks 20 years since a helicopter pilot flying over central Australia spotted the outline of a giant man drawn into the earth.
The 4.2km (2.5 miles) tall figure, on a remote plateau in South Australia, is often thought to depict an Aboriginal hunter.
Dubbed Marree Man after a nearby town, it is one of the world's largest designs to be etched into the ground.
But mystery surrounds who created it - and why.
Earlier this week, Australian entrepreneur Dick Smith offered a $5,000 Australian dollar (£2,800; $3,700) reward for any information about the artwork's origins.
"How has it been kept secret for 20 years?" he said on the Australian Broadcasting Corporation (ABC) on Monday.
Marree Man has been a subject of fascination since its discovery in the desert about 700km north of Adelaide.
It has gained popularity on tourism flights because it is too large to be viewed from the ground.