Secret History (1991)

Season 14: Episode 4

With access to the first nationwide survey and state-of-the-art drones mapping the Great Wall of China, this documentary offers new evidence that reveals its extraordinary magnitude, rewrites its millennia-long history and decodes its complex signals system. Using cutting-edge chemistry, the programme also finds out what has kept the wall standing for so long - a simple cooking ingredient found in kitchens across the world.