Great Wall of China

http://www.straightdope.com/classics/a2_092.html

Tom Burnam, author of More Misinformation (1980), quotes a letter from astronaut Alan Bean on the subject:"The only thing you can see from the moon is a beautiful sphere, mostly white (clouds), some blue (ocean), patches of yellow (deserts), and every once in a while some green vegetation. No man-made object is visible on this scale. In fact, when first leaving earth's orbit and only a few thousand miles away, no man-made object is visible at that point either."