China's Great Wall stretches from the banks of the Yalu River to Gansu Province - the place in China's far west where the "barbarian" territories of Central Asia were reconded by the Chinese to begin. Estimates of the Wall's length vary. But the distance from the Wall's eastern end in Liaoning Province to its western end in Gansu is usually put at about 6,000 kilometers -- as the crow flies. People, however, can't fly; and to actually walk from the length of the Wall's twisted path through northern China over the many mountain peaks and down the valley's between them would be further. One estimate places the actual length of the Wall (if it could be stretched out into a straight line) at about 15,000 kilometers, or 9,000 miles. Over the centuries, armies have been stationed along the length of the Wall to provide early warning of efforts to invade China.