You just knew we'd be visiting Stonehenge at some point during this list! This stone circle, which can be found in the south west of England, is one of the best-known historical sites in the entire world. Its fame hasn't brought it any detailed understanding, though.
Most people now agree that the stones are a form of ancient calendar, which tracked the movement of the Sun and the Moon. What we don't know is how on earth people managed to move the stones into position 5000 years ago.