From Halopedia, the Halo wiki
The Zeta Hydronis Secundus system was a planetary system within the Forerunner ecumene. At the height of Forerunner power, the planetoids in the system hosted thriving colonies and impregnable fortresses, but during the Forerunner-Flood war the system fell before the parasite. In the post-Covenant War era, the UNSC found the system, designating Site C to one of a handful of intact Forerunner structures that remained.
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