Actor Jason Watkins and historian Tracy Borman investigate the final mystery of Stonehenge - the secret of the altar stone, a six-ton slab at the heart of Stonehenge that has puzzled archaeologists for centuries. It's the only stone that lies flat on its side, a different type from all the others, and whose origins are still unknown. In a TV first, Jason and Tracy reveal new research from Aberystwyth University about the origins of the stone