The Marree Man [wiki] is 4.2 km tall figure of a guy throwing a stick carved into the earth of South Australia. It was spotted by a charter pilot from the air in 1998.
The lines of the figure was made with a plough attached to a tractor, but no one knows who made it or why. Perhaps the maker just wanted to make the largest manmade artwork in the world.
Link - via Scribal Terror