The locals of West Dunbartonshire, Scotland are so used to the Cochno Stone that children used to play on it. Thankfully it's now protected from such use, so the markings on the stone aren't worn away. That's good, because we're still trying to work out what they mean.
The stone is approximately 5,000 years old, and is one of the earliest surviving examples of Bronze Age cup and ring carvings. Theories about its content range from a star map to pure and simple decoration.