The Great Wall of China is one of the largest construction projects ever, it is about 5,500 miles long and runs from east to west, from Mount Hu to the Jiayu Pass. As a side note I have never been to China so I have never seen it first hand, I sure would like to though. About 30% of it is natural barriers the rest 70% is actually constructed walls.
The main wall was built over a span of nearly two hundred years and it actually consists of many walls. It was built as a defense against northern invaders, despite which China was conquered by Genghis Khan, which just goes to show a wall will not stop a determined adversary (OK, the wall wasn't completely finished when Genghis invaded).
Today a good part of the wall of China is in ruins, but it still stands as an outstanding example of human engineering. The wall is a UN World Heritage site, it is visited every year by about 10 million tourists so it is a good revenue source.
Now there is a common belief among many people that the great wall is the only human built structure that is visible from the moon. Now the wall as I said is over 5,500 miles long but it is only at the most 20 feet wide, I always wondered why it could be seen from the moon and you couldn’t see say one of the US 8 lane highways (just as an example, I know there are way wider highways) in which each lane is 12 feet wide, so just one side of the highway is nearly 50 feet wide.
Well the answer is you can’t see it, the following is a quote from a NASA article:
“It has become a space-based myth. The Great Wall of China, frequently billed as the only man-made object visible from space, generally isn't, at least to the unaided eye in low Earth orbit. It certainly isn't visible from the Moon. ”