Defensive wall, northern China. One of the largest building-construction projects ever carried out, it runs (with all its branches) about 4,500 mi (7,300 km) east to west from the Bo Hai (Gulf of Chihli) to a point deep in Central Asia. Large parts of the fortification date from the 7th to the 4th century BC. In the 3rd century BC the emperor Shihuangdi connected existing defensive walls into a single system fortified by watchtowers. These served both to guard the rampart and to communicate with the capital, Xianyang (near modern Xi'an) by signal—smoke by day and fire by night. Originally constructed partly of masonry and earth, it was faced with brick in its eastern portion. It was rebuilt in later times, especially in the 15th and 16th centuries. The basic wall is about 23–26 ft (7–8 m) high; at intervals towers rise above it to varying heights. It was designated a UNESCO World Heritage site in 1987.
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The Great Wall (also called Coma Wall), sometimes specifically referred to as the CfA2 Great Wall, is the second largest known super-structure in the Universe (the largest being the Sloan Great Wall). It is a filament of galaxies approximately 200 million light-years away and has dimensions which measure over 500 million light-years long, 300 million light-years wide and 15 million light-years thick. It was discovered in 1989 by Margaret Geller and John Huchra based on redshift survey data from the CfA Redshift Survey.
It is not known how much farther the wall extends due to the plane of the Milky Way galaxy in which Earth is located. The gas and dust from the Milky Way (known as the zone of avoidance) obscures the view of astronomers and have so far made it impossible to determine if the wall ends or continues on further than they can currently observe.
The Standard Model cannot account for such large structures, so in the actual cosmology it is hypothesized that such structures as the Great Wall form along and follow web-like strings of dark matter. It is thought that this dark matter dictates the structure of the Universe on the grandest of scales. Dark matter gravitationally attracts normal matter, and it is this normal matter that astronomers see forming long, thin walls of super-galactic clusters.