The largest man-made structure at the time of its completion in the 17th century, China’s Great Wall spans 5,500 miles (8,850 km) across northern China, from the Korean border west into the Gobi desert. It actually started out as a series of smaller walls built by Chinese warlords in the seventh century B.C. to defend their individual lands. As China became a united empire, the walls were joined and fortified over the next 2,000 years to repel enemies, particularly the Mongolian and Manchu armies. Built from countless bricks and stones and guarded by watchtowers, the Great Wall is an engineering marvel of the ancient world. You can’t spot the Great Wall from Earth orbit with the naked eye (the wall tends to blend in with the mountainous landscape). Astronauts can see all sorts of other man-made objects – roads, cities, dams, and even the Great Pyramids – from their spaceship portholes.
Picture Credit : Google