Where: Wiltshire, England

Built: Between 2500 and 2000BC

Added to Unesco's World Heritage Site in 1986, Stonehenge is a familiar image to millions around the world. And yet, explanations as to why the site was built are still shrouded in mystery. Linked with the druids, Merlin the wizard and the Devil himself, the structure is a remarkable feat of engineering considering the date of its construction.

"If we take the history of the oldest cathedral in Christendom and double it, we are still a thousand years short of the time Stonehenge took to evolve" - Rodney Castleden, geographer and geomorphologist