Installation 05, or 'Delta Halo', being 'primed' in Halo 2
The Halos are giant weapons of mass destruction on a galactic scale created by the Forerunners. They resemble massive rings (hence the name "Halo") with the inner ring surface bearing M class planetary atmosphere and landscapes. On the surface the Halo installations are indistinguishable from other inhabitable planets except for the persistent ring one sees in the sky. The Halo Installation 04 was found in the beginning of Halo: Combat Evolved. The Covenant believed it to be a holy place, and thought that only by activating it could they be saved and experience "The Great Journey". Cortana and the Master Chief discovered that this was not true when 343 Guilty Spark, the monitor of the Halo, told them otherwise. They learned that the Halos were in fact created with the sole purpose of eliminating the Flood. The Halos can only be activated by using the Index, which is found in the Library of every Halo. Only humans can activate the Halos, for they are the chosen species by the Forerunners.
 The Different Halos
There are seven different Halos in the galaxy. We have seen four, with one being a replacement for another.
 Installation 03
The monitor is 049 Abject Testament.
 Installation 04
Installation 04 was the first Halo seen by the humans. It was first and last seen in Halo: Combat Evolved. It was destroyed when the Master Chief detonated the Pillar of Autumn's fusion reactors. Installation 04 is also commonly called "Alpha Halo".
 Installation 04 (II)
Installation 04 was replaced during Halo 3 by the Ark. It was first seen at the end of the level The Covenant. This Halo was also destroyed by the Master Chief, this time by activating it prematurely. Activating it before it was ready allowed the local Flood infestation to be destroyed without doing damage on a larger scale like the Halos normally do. It is also believed that it may have killed Gravemind.
 Installation 05
Installation 05, also known as "Delta Halo" was the Halo seen on the game Halo 2. It was discovered by the Prophet of Regret, who was followed by the humans. At the end of Halo 2, the Brute Chieftain Tartarus captured Miranda Keyes and forced her to place the Index into the control center, activating Installation 05. It was Sgt. Johnson, along with a few high ranked Elites, and the Arbiter who killed Tartarus and had the Index pulled out of the Halo before it could fire.
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