Installation 05
From Halopedia, the Halo wiki
Summary
Installation 05 or Delta Halo is where most of Halo 2 is played in. It's pretty much the same as Installation 04 with regards to the terrain. It is also where the Flood puppetmaster Gravemind resides. The Monitor of this Installation is 2401 Penitent Tangent. Like all other Installations this is also used to contain Flood. At the end of Halo 2, Tartarus almost activates this Installation but Sergeant Avery Johnson and the Arbiter kill him before he succeds. A beacon was sent from this Installation to the Ark. At the end 343 Guilty Spark explains to Sgt. Jonson that the Halos can only be activated from the Ark which possibly could be on Earth.
