Great Wall of China
The Great Wall of China was a structure built in China on Earth to, as Kathryn Janeway pointed out in 2375, "keep people out". It was completed by the first Qin emperor in the 3rd century BC, who connected the walls built by a previous dynasty. He did so to prevent an invasion by nomads to the north. The full wall was 2,400 kilometers long, with a median width of 3.8 meters. 

According to Neelix and Janeway, before the 22nd century, it was one of the only man-made objects that could be seen from Earth orbit with the naked eye. ()

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de:Chinesische Mauer
es:Gran Muralla China
fr:Grande muraille de Chine
Earth structures