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 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 Captain 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. (VOY: "11:59")
Background
The statement that the Great Wall is the only man-made object that can be seen from space is a well-known false urban legend. [1] [2] According to several NASA astronauts, the Wall itself is not visible from medium to high orbits (180 miles from the surface). While it may be visible from lower orbits (around 130 miles from the surface), other man-made objects are clearly visible as well at that height as well, even more so than the Great Wall. NASA space shuttle astronaut Jay Apt has stated that "Although we can see things as small as airport runways, the Great Wall seems to be made largely of materials that have the same color as the surrounding soil. Despite persistent stories that it can be seen from the moon, the Great Wall is almost invisible from only 180 miles up!" ("National Geographic", Nov 1996) It could be that the Great Wall of the Star Trek Universe is significantly larger than that of the real world; it is also possible that this urban legend persists into the 24th century (at least as to pre-22nd century man-made objects and the observability of the Earth from space with pre-22nd century technology) and that Neelix and Janeway have both fallen victim to it.
Neelix was also in error regarding the length of the wall; it is actually almost 6400 kilometers long. Because the dialog is exactly 4000 kilometers off, it is possible that Ethan Phillips misread the script and the correction deemed too minor to re-film. Alternatively, it is possible 4000 kilometers of the wall was demolished between now and 2375.