Summary For years, historians thought that Incan khipus - a series of knotted cords on a line - were simple counting devices, like a woollen abacus. But one investigator believes khipus are actually a three-dimensional system for record keeping. By examining an ancient khipu with a multi-spectral scanner, she hopes to crack the code of the Incas and reveal their secrets. Then, journey to the other side of the world where Vikings spread their terror across the high seas. Considered fearsome warriors with superior navigational skills, they reached North America hundreds of years before Columbus. Historians still don't know how they did it without a magnetic compass. Their sagas mention the existence of magic crystals - stones which, according to legend, guided them across the seas. Now, one woman, a Viking descendant and an expert on bird migration, investigates the legend of these mysterious sunstones.

• Documentary
• History