The Great Wall
The Great Wall is a magnificent construction, but you can't see it *by naked eye* from outer space (really!). It originated as a collection of separate fortifications, that were built of pounded earth by the warring states in the late Zhou period. These defenses were consolidated during the Qin dynasty, and alternately extended, neglected, repaired, and rebuilt at many different times thereafter. Beginning with the Ming dynasty, pounded-earth was replaced by stone and brick construction.
The part of the Great Wall seen in this photo dates from the Ming dynasty, although it has been heavily restored in modern times. It is located at Badaling, 68 km (42 miles) northwest of Beijing.
The Ming recension of the Great Wall extends four thousand miles, driving westward from Bohai on the northest coast, above Beijing, and from there all along the northern boundary of China, until it eventually peters out at Lop Nur in Gansu, halfway along the Silk Road (map). The longest defensive structure in the world, the Great Wall demarcated the boundary between China and the "northern barbarians" (compare: Hadrian's Wall), against whom it was, at best, only intermittently effective: peoples such as the Jurchen Jin, the Mongols (Yuan Dynasty), and the Manchus (Qing Dynasty) had no difficulty in penetrating it.