Installation 05 (Earth-2042)

"This installation has a successful utilization record of 1.2 trillion simulated and one actual. It is ready to fire on demand."

- 2401 Penitent Tangent

Installation 05, also known as Delta Halo, is one of the massive Halo fortress worlds built by the Forerunners. The ring was formerly maintained by the Metarch-series monitor droid 2401 Penitent Tangent, until his lax oversight led to the Flood's escape from their containment facilities. The Halo orbits Substance, a gas giant and the fourth planet in orbit in the Substance system, located in the Norca sector of the Mid Rim.

Source

 * https://www.halopedia.org/Installation_05