Installation 01

Installation 01 is one of the seven Halo Rings. Very little is known about it and it has yet to be directly observed.

In 2552, during the Battle of Installation 05, a signal was sent to this Halo from Installation 05, putting it on standby mode. A readout from Installation 05's Control Room showed that Installation 01 was fully prepared to fire on demand.

A more comprehensive readout was later observed in the Ark during the Battle of the Ark by the Arbiter, Master Chief, and Prophet of Truth, once again displaying that Installation 01 was on standby mode, waiting to receive the signal to fire. However, the Master Chief deactivated the installation before it could be fired.

Trivia
If you look at this ring it looks like it may have probably been taken over by the Flood.