Threshold

Summary
Threshold is a large gas planet. Before its destruction, Installation 04 orbited between it and its moon Basis. In Halo 2, the Heretics took refuge on a Forerunner Gas Mine in its atmosphere before the Arbiter and his Special Ops troops dropped it into Threshold's depths.

Trivia
In Halo 1, the Assault Rifle has a small digital display that shows how much ammunition is left in the clip as well as a indicator that points towards Threshold, acting like a compass.