Installation 05

Installation 05, or Delta Halo, is where most of Halo 2 occurs. 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 succeeds. A beacon was sent from this Installation to the Ark, moving all installations (Halos) to stand by status. At the end 343 Guilty Spark explains to Sgt. Jonhson and Commander Keyes that the Halos can be remotely activated from the Ark, which possibly could be on Earth.

Infestation
On Installation 05, An infestation has already broken out when the Chief gets there. On the level regret, if you look out to the far edge of the lake (the area that isnt covered high with mountains), you can just see the outer wall of the Quarantine Zone.

It is likely that the Flood escaped from this installation by some form of energy leak, causing the Flood to wake up from stasis.