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The '''Great Wall of China''' is any of several extended frontier fortifications in northern China. These were constructed in ancient times as barriers against invading armies. The original wall was built in 220–206 BC by [[Qin Shi Huang]], the emperor who unified China. In modern times, this wall is no longer recognizable from the the ground, but it has been mapped using satellite photography. The modern form of the Great Wall was built during the [[Ming dynasty]] (1368 - 1644). The claim that the wall can be seen from space is an urban legend.
The Great Wall winds some 2,400 km (1,500 mi) along the edge of the Mongolian plateau from Gansu Province in the west to the Yellow Sea in the east. Its width ranges from 4 to 12 m (12 to 40 ft) and its height from 6 to 15 m (20 to 50 ft). It makes possible much more effective military defense of China from invaders. It is 4,000 miles long and more than 20 feet tall.