The Great Wall of China was a structure built in China on Earth to, as Neelix pointed out in 2375, "keep people out". It was completed by the first Qin emperor in the 3rd century BCE, who connected the walls built by a previous dynasty. He did so to prevent an invasion by nomads to the north. The full wall is 2400 kilometers long, with a median width of 3.8 Earth meters.
As Neelix also pointed out, before the 22nd Century it was one of the only man-made objects that could be seen from Earth orbit with the naked eye.(VOY: "11:59")
- According to several NASA astronauts, the Wall itself is not visible from medium to high orbits (180 miles from the surface), but may be visible from lower orbits (around 130 miles from the surface). One major issue, according to NASA space shuttle astronaut Jay Apt, is that "the Great Wall seems to be made largely of materials that have the same color as the surrounding soil."
- Neelix was in error regarding the length of the wall; it is actually almost 6400 kilometers long. Because the dialogue is exactly 4000 km. off, it is possible that Ethan Phillips misread the script and the correction deemed too minor to refilm. Alternatively, it is possible 4000 km. of the wall was demolished between now and 2375.