Installation 05

Summary
Installation 05 is the official name of the ring-shaped space station called Halo (or Delta Halo in full) in Halo 2

The Station
In Halo 2, Installation 05 is one of the Halos built by the Forerunners to contain and study the Flood. Unlike Installation 04, the Flood seems to have been loose for a very long time, and only made their presence known after the Arbiter shut down the Library's shields. Deep beneath the Library, lies the Flood Gravemind. At some point, the Gravemind captured 2401 Penitent Tangent, the Monitor of Installation 05, and thus the Forerunner structures absent of Constructors are in a state of disrepair.

According to Penitent Tangent, the Halo has fired 1.2 trillion times in simulation and once for real. The Halo almost fired again at the end of Halo 2, but stopped when Miranda Keyes removed the Index. The Halo did send out a signal putting all other Halos on standby, to be activated from the Ark.