It's the most extraordinary feat of engineering in history, and one of the most iconic man-made structures on the planet - the Great Wall of China, stretching thousands of miles across barren deserts and treacherous mountains before finally plunging into the sea. But why did the Chinese go to such staggering lengths to build it, and what are the secrets that have enabled it to survive for over 2,000 years? Now, ground breaking science is re-writing its complex history and de-coding its mysterie...