Ploughed into the Australian outback in 1998 (by persons still unknown), the Marree man is a 4km figure of an aboriginal. It’s faded into the desert quite a bit now but the shape (and size!) is still quite apparent. Wikipedia says that it was probably created using GPS and earthmoving equipment. There’s a good page here which discusses the possible reasons for creating the artwork. Many thanks to Ken Mortimer, who it seems was the only person to submit this!