A crude carving that archaeologists feared was a graffiti vandalism has turned out to be an ancient work of art.
A huge waterfall was found carved into the rock, along with 15,000 other images.
Experts initially thought the mass male member was the work of modern-day mischief makers.
But now they estimate that the carvings are about 5000 years old.
The prominent penis is part of the Marra Wonga site in Australia.
Thousands of carvings at the Outback Rock Shelter are the work of ancient local artists.
Many images tell the Star Dreaming story of the Seven Sisters, where a shapeshifter transforms into a boomerang-throwing Falls.
Professor Paul Taccone from Griffith University is cataloging the art.
He said: “In the story, the sisters are pursued by a powerful ancestor known as Vatanori, who is often associated with the constellation Orion.
“At one point, he goes underground and emerges as a giant penis and throws a boomerang at the sisters, which we see very clearly in this panel.”
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Other artifacts inside the cave, such as six-toed feet, show that it was used to tell dream stories.
Experts used lasers and drones to document the site and said it may have been a teaching site.
Professor Taccone told the Australian Journal of Archaeology: “There is no other site in Australia that has the art of telling a story from one end of the shelter to the other.”