Measuring more than 4,000 miles (6,400 km), the Great Wall is the longest structure ever built by humans. It is also the largest human-made structure in terms of mass and surface area.
Not Just One Wall
The Great Wall is actually a series of walls that were built around individual regions, then joined into a single structure over time. In 221 BC, Emperor Qinshihuang, who had defeated six rival states and unified China, decided to link the defensive walls built by those he conquered.
A Lengthy Construction Project
After this initial link-up, Chinese leaders continued to extend the wall and rebuild sections that deteriorated. Construction continued through China's feudal history into the 17th century.
More Than a Wall
The Great Wall also includes forts, watchtowers, signal towers and other structures that were used for storage, administration and housing soldiers. Peasants who died working on construction of the wall--possibly as many as a million--are entombed within the wall.
A Mixture of Construction Types
The earliest portions of the Great Wall were built of earth, stone and wood. Later, bricks, tiles and lime were used because they could bear more weight and made construction quicker.
The Great Wall and Outer Space
The theory that the Great Wall can be seen from the moon dates to at least 1938, but it was disproved when astronauts landed on the moon and could see nothing of the Earth but a blue and white sphere. A November 24, 2004, photo taken from the International Space Station, however, does appear to show a small section of the Great Wall.