From Halopedia, the Halo wiki
Along with Padpad, Buwan was hollowed out by the Covenant's tithe fleets, the resources of the moons stripped to feed the assembly forges of the Holy City of High Charity. The moon was the site of a Covenant hydro-processing center that harnessed Buwan's waters to refuel and power capital ships. The facility was abandoned following the splintering of the Covenant, though it continues to operate via automated subsystems. Office of Naval Intelligence scans indicate that the moon was completely abandoned after the Human-Covenant War, though ONI continues to monitor the Tala system.
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