3. The great wall
The Great Wall remains today as it has been for more than twenty centuries the only work of men that is visible from the moon.
The construction of the wall began in the seventh century B.C. When separate feudal states in northern China built barriers against invasions by neighboring states along their borders.
The wall traverses plains and mountains being at some points 1300 meters above sea level. The wall averaged 7.8 meters in height and 5.8 meters at the top.
