The Great Wall of China ( literally "10,000 Li long wall"), is an ancient Chinese fortification built circa 200 BC and greatly strengthened from the 14th century until the beginning of the 17th century, during the Ming Dynasty, in order to protect the Ming empire from raids by the Mongols and Turkic tribes. It was preceded by several walls built since the 3rd century BC against the raids of nomadic tribes coming from areas in modern-day Mongolia and Manchuria. The Wall stretches over a formidable 6,352 km (3,948 miles), from Shanhai Pass on the Bohai Sea in the east, at the limit between China proper and Manchuria, to Lop Nur in the southeastern portion of Xinjiang Uygur Autonomous Region.
Because many people died while building the wall, it has obtained the gruesome title, "longest cemetery on Earth" or "the long graveyard." Possibly as many as one million workers died building the wall.
The Great Wall seen today was built during the Ming Dynasty, starting around the year 1368 and lasting till around 1640. This new wall was built on a grand scale with longer lasting materials (solid stone used for the sides and the top of the Wall) than any wall built before. The primary purpose of the wall was to make it difficult for semi-nomadi people outside the wall (such as the Mongols under Altan Khan and the Oirats under Esen Taiji) to raid into China with their horses or return with stolen property.
The last great wall of the Ming really was a military fortification of some strength. However, military historians are generally dismissive of the net value of this great wall. It was astonishingly expensive to build, maintain and garrison. The money the Ming spent on the wall could have been spent on other military capabilities such as European style artillery or muskets. The fact remains that the great wall was of no help at all in the Ming dynasty's fall.
There is a longstanding disagreement about how visible the wall is in space. The notion of its visibility from outer space greatly predates manned space flight.
In fact, from near-earth orbit, it is barely visible, and only under nearly perfect conditions. And it is no more conspicuous than many other manmade objects. The state-run China Daily newspaper concluded that the Great Wall can be seen from space with the naked eye, under favorable viewing conditions, if one knows exactly where to look.