From Halopedia, the Halo wiki
Shadow Perimeter Three the a colloquial term for an area on Luna. The term refers to the system of tunnels and surface tubes that run across Mare Nubium from the Academy at Mare Nubium to Asimov Center. The "shadow" part of the name describes the shadows cast by the crater walls.
In 2492, Preston Jeremiah Cole, then a cadet, was ordered to guard the perimeter alongside cadets Parkins, Haverton, and Tasov. Cole later speculated that the reason for their orders was either to protect against unauthorized civilian incursions or to check on rumors of military supplies illegally moving into civilian sectors.
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