From Halopedia, the Halo wiki
Site C is the United Nations Space Command's designation for a Forerunner installation within the Zeta Hydronis Secundus system. At the height of the Forerunners' power, the planetoids in the system hosted thriving colonies and impregnable fortresses, but during the Forerunner-Flood war the system fell before the parasite. When the system was discovered by the UNSC after the Human-Covenant War, Site C was one of a handful of intact Forerunner structures that remained.
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