What interests me about the Great Wall is that a few years ago I learned that the wall in this and other postcard pictures is not the original wall. The wall built by China's first emperor, in the third century B.C., was largely made of earth held together by mats, a simple but very strong building material. The watchtowers were behind the main structure, rather than part of it. 1,600 years later, it had crumbled to a sandy ridge, prompting the Ming dynasty emperors to rebuild the wall; they covered the earthern structure with stone, and made it nearly twice as high as it had been before. That's the Great Wall we're familiar with.
However, most tourists only see the part of wall near Beijing. The rest is a threatened monument, endangered by time, wind erosion from the Gobi desert, and plundering local peasants who sometimes don't even know what they're taking bricks from. A 2003 study discovered that less than 20 percent of the Ming dynasty wall is intact, and a third of it has vanished completely.