There is a stone at the centre of Stonehenge that almost nobody photographs. Five metres long, flat on its face, half buried, crushed under a fallen sarsen that toppled onto it thousands of years ago. Millions of visitors have stood over it without knowing it was there. For a hundred years the textbooks said they knew where it came from, and for a hundred years they were wrong. Nobody dug it up to find out. The answer came from grains of mineral smaller than sand, dated in a laboratory and run through software that had every rock in Britain to compare them against. And what the machine matched them to should not have been possible in 2600 BC.