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Though one of the oldest and largest man-made structures in the world, the Great Wall of China is also one of the least known. Exploring it has become the lifetime obsession of British historian William Lindesay, who has lived in China for 20 years. In this 50-minute special, Lindesay travels from one end of China to another to show us how and why the Wall was built, how its construction differs according to region – and how it is really many walls built by different dynasties, and not one single Wall.