The Great Wall of China

The Great Wall of China (simplified Chinese: 长城; traditional Chinese: 長城; pinyin: Chángchéng; literally "long city/fortress") or (simplified Chinese: 万里长城; traditional Chinese: 萬里長城; pinyin: Wànlǐ Chángchéng; literally "The long wall of 10,000 Li (里)"[1]) is a series of stone and earthen fortifications in China, built, rebuilt, and maintained between the 5th century BC and the 16th century to protect the northern borders of the Chinese Empire from Xiongnu attacks during the rule of successive dynasties. Several walls, referred to as the Great Wall of China, were built since the 5th century BC. One of the most famous is the wall built between 220–206 BC by the first Emperor of China, Qin Shi Huang; little of it remains; it was much farther north than the current wall, which was built during the Ming Dynasty.[2]
The Great Wall stretches over approximately 6,400 km (4,000 miles)[3] from Shanhaiguan in the east to Lop Nur in the west, along an arc that roughly delineates the southern edge of Inner Mongolia, but stretches to over 6,700 km (4,160 miles) in total.[4] At its peak, the Ming Wall was guarded by more than one million men.[5] It has been estimated that somewhere in the range of 2 to 3 million Chinese died as part of the centuries-long project of building the wall.[6]
Visibility from the moon
Popular beliefs ranging from Ripley's Believe It or Not!'s cartoons from 1930s, which claimed that the Great Wall is "the mightiest work of man, the only one that would be visible to the human eye from the moon," to Richard Halliburton's 1938 book Second Book of Marvels which makes a similar claim, have persisted, assuming urban legend status, and sometimes even appeared in school textbooks. Arthur Waldron, author of The Great Wall of China: From History to Myth, has speculated that the belief might go back to the fascination with the "canals" once believed to exist on Mars.
One of the earliest known reference to this myth appears in a letter written in 1754 by the English antiquary William Stukeley. Stukeley wrote that, "This mighty wall of four score miles in length (Hadrian's Wall) is only exceeded by the Chinese Wall, which makes a considerable figure upon the terrestrial globe, and may be discerned at the moon."[11]
The Great Wall is a maximum 9.1m (30 ft) wide and is about the same color as the soil surrounding it. Based on the optics of resolving power (distance versus the width of the iris: a few millimetres for the human eye, metres for large telescopes) an object of reasonable contrast to its surroundings some 70 miles in diameter (1 arc-minute) would be visible to the unaided eye from the moon, whose average distance from Earth is 384,393 km (238,857 miles). The Great Wall is of course not a disc but more like a thread—it can be seen from much further than would be possible if it were simply a 30 foot disc. Still, the apparent width of the Great Wall from the moon is the same as that of a human hair viewed from 2 miles away. To see the wall from the moon would require spatial resolution 17,000 times better than normal (20/20) vision.[12] Not surprisingly, no lunar astronaut has ever claimed seeing the Great Wall from the moon.
Incidentally, if one could have seen the Great Wall from the moon, one ought to be able to see most of the roads in the world as well, given that they in total length far surpasses that of the great wall, and commonly are even wider. This is again not the case.

Visibility from low earth orbit
A more controversial question is whether the Wall is visible from low earth orbit, i.e an altitude of as little as 100 miles (160 km). NASA claims that it is barely visible, and only under nearly perfect conditions; it is no more conspicuous than many other man-made objects.[13] Other authors have argued that due to limitations of the optics of the eye and the spacing of photoreceptors on the retina, it is impossible to see the wall with the naked eye, even from low orbit, and would require visual acuity of 20/3 (7.7 times better than normal).[12]