Chaco Canyon in New Mexico contains the spectacular ruins of a vanished civilisation. Great masonry buildings, built in a near-desert, bear witness to an American people who have now vanished without trace. They left behind elaborate walled cities, complete with living quarters, vast storage rooms and large round underground chambers called 'kivas', in which they worshipped. Between their towns they constructed roads; they also had a long-distance signalling system. At the height of their power the people of Chaco controlled an area of some 40,000 square miles. But why did the civilisation fall? Is there a message for us today in what happened 600 years ago? Recent archaeological excavation provides us with some of the answers.