The Great Wall of China stretches about 5,500 miles and passes through 10 Chinese provinces.
The wall was built primarily to keep out the Mongols and other invading tribes from the north.
More than 400 world leaders have visited the Great Wall of China in the past 50 years, including Queen Elizabeth, Margaret Thatcher, Nelson Mandela and presidents Richard Nixon, Ronald Reagan and George H.W. Bush.
Erosion and visitors have taken their toll on the wall. Today, less than half of it remains standing.
Many people think that the Great Wall of China is the only man-made object visible from space. Actually, NASA says that the wall is extremely difficult to see from orbit, even under optimum conditions.