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Diving deep into the secrets of the Great Wall

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BEIJING - The Great Wall of China is one of the world's most famous landmarks, but much of it is still being explored. NBC's Ian Williams joined noted Great Wall historian, William Lindesay and Steven Schwankert of SinoScuba to take a unique look at one hidden section of the wall, diving down into a vast lake that submerged the wall when an entire valley was flooded decades ago.