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An engineering feat rarely matched in the history of the world, the 1500-mile-long Great Wall of China stretches from the shores of the Pacific to the Gobi Desert. It is such a prominent structure on earth that astronauts say they can see it with the naked eye from space. The 2000-year-old Great Wall was originally constructed to shield the ancient empire from invaders, but today it is regarded as a national symbol and attracts hordes of tourists from around the world.