The Great Wall of China is incredibly long, and was originally built for military purposes. Check out this HowStuffWorks podcast to learn more about the Great Wall.
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Percy Fawcett, Manuscript 512, and a Lost City in the Amazon, Pt. 2
37:10

Percy Fawcett, Manuscript 512, and a Lost City in the Amazon, Pt. 1
34:05

SYMHC Classics: Swill Milk
39:22