Archaeologist George Nash explores five of Western Europe's most remarkable rock art sites.
Today he risks his neck in Northern Portugal's Coa Valley to see Iron Age carvings of beasts and warriors. But why are they engraved directly on top of earlier art in such dangerous places? And what drives modern day graffiti artists to risk their lives to the same end?
Producer: Chris Eldon Lee
A Culture Wise production for BBC Radio 4.