While in a low orbit, a viewer can see the Great Wall of China (pictured here in a satellite image) from space, but it is not unique in that regard. From a low orbit of the earth, many artificial objects are visible on the earth, not just the Great Wall of China: highways, ships in the sea, dams, railroads, cities, fields of crops, airports, and even some individual buildings. As to the claim that it is the only man-made object visible from the moon, Apollo astronauts reported that they could not see any man-made object from the moon, not even the Great Wall.