Great Wall of China
The Great Wall of China is a long wall in China. It was constructed in ancient times as a barrier against invading armies. This was unsuccessful as the Mongols simply bribed the guards. It is 4,000 miles long and more than 20 feet tall.
Work started in 500 BC under Qin Shi Huang, connecting older walls which were constructed by individual states. Most of the work was done in the Ming Dynasty. Towers were built; originally as watchtowers, but later cannons were added.
The Great Wall of China is not the only man-made structure visible from space, or the Moon. This legend probably originated with Richard Halliburton's 1938 "Second Book of Marvels". It has been disputed whether it is even visible from low earth orbit. A 2008 article in Scientific American stated: "The unglamorous truth is that the wall is only visible from low orbit under a specific set of weather and lighting conditions. And many other structures that are less spectacular from an earthly vantage point—desert roads, for example—appear more prominent from an orbital perspective." In 2005 an American astronaut (of Chinese descent) succeeded in photographing the Wall. 
There is no truth to the notion that there are few human structures visible from space; Cecil Adams comments:
Any number of man-made structures can be seen from space, provided we mean "structure" to mean "anything built." Many of these are things that look like long, straight lines when seen from far off, such as highways, railroads, canals, and of course walls. If the orbit is low enough you can see even more, I have here a photo of Cape Canaveral taken during the Gemini V flight in which the big Launch Complex 39, used for the Apollo missions, is clearly visible. Another photo of the Nile delta, taken from a height of 100 miles, shows an extensive road network. Gemini V astronauts Gordon Cooper and Charles Conrad were able to spot, among other things, a special checkerboard pattern that had been laid out in Texas, a rocket-sled test in New Mexico, and the aircraft carrier that would later pick them up in the Atlantic, along with a destroyer trailing in its wake.
As to whether the Wall can be seen from space, Apollo 12 astronaut Alan Bean says, "The only thing you can see from the Moon is a beautiful sphere, mostly white, some blue and patches of yellow, and every once in a while some green vegetation. No man-made object is visible at this scale." 
According to NASA director Adnan Lulla, "A lot has been said and written about how visible the wall is. In fact, it is very, very difficult to distinguish the Great Wall of China in astronaut photography, because the materials that were used in the wall are similar in color and texture to the materials of the land surrounding the wall -- the dirt." 
The Great Wall was listed as World Heritage by the UN in 1987. Although long sections of the wall are now in ruins or have disappeared, it is still one of the more remarkable structures on earth and China's best-known monument.
- ↑ Hucker, Charles O. China's Imperial Past: An Introduction to Chinese History and Culture. Stanford UP, 1997.
- ↑ Susan Wise Bauer, The History of the Ancient World
- ↑ http://factsanddetails.com/china.php?itemid=27&catid=2&subcatid=1
- ↑ Scientific American, Is China's Great Wall Visible from Space? Though it stretches for some 4,500 miles, the ancient Chinese fortification is not as visible from orbit as modern desert roads, by Mara Hvistendahl, February 21, 2008
- ↑ BBC News, Shanghai Great Wall visible in space photo, by Francis Markus, 19 April, 2005 BBC News report of "China Daily" article 4/19/2005
- ↑ Is the Great Wall of China the only man-made object you can see from space? Cecil Adams, The Straight Dope
- ↑ NASA, China's Wall Less Great in View from Space, 05.09.05
- ↑ http://www.nasa.gov/vision/space/workinginspace/great_wall.html
- ↑ http://www.thegreatwall.com.cn/en
- Great Wall of China Encyclopaedia Britannica.