From Halopedia, the Halo wiki
At some point a thermal reactor located at the Pilvros Municipal Support Facility was failing. The facility attempted to flood the reactor and save it but that failed, and as a result the city was evacuated. Before the reactor went critical, a team was dispatched to the site to retrieve full environment scans of the facility for study in hopes of preventing similar accidents in the future. These scans were later used to produce a simulated environment for War Games exercises aboard UNSC Infinity. In 2558, the Pilvros Reclamation Facility's overseer AI, Dillan, defected and joined Cortana's Created.
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