Northern Ireland. Antrim county. Formed 40 million years ago  the Giant's Causeway is due to a basalt flow in a limestone valley which was covered by a layer of clay  a layer which cools it; as it cooled  the basalt burst into organ-like shapes. The legend says that it was the Irish giant Fionn Mac Cumhaill who threw stones into the sea to challenge his Scottish counterpart  Benandommer. The site has been on the UNESCO World Heritage List since 1986.