Threshold gas mine

In Threshold's atmosphere, there is a Forerunner facility built to mine the gas from the planet. In the book, The Art of Halo, it is stated that this is where the Forerunners first encountered (or perhaps even created) the Flood. Hanging from a support superstructure higher in the atmosphere, the mine is shaped somewhat like a grappling hook, with one center shaft and many legs bending down and inwards.

After the Forerunners encountered the Flood, they retrofitted the mine to research the possible offensive and defensive measures against them. If the Flood were to escape, the mine would simply be dropped into the gas giant. In fact, this was the station's eventual fate: the Arbiter cut the cables holding up the station, dropping it into Threshold's depths.

The Covenant Heretics made their headquarters out of the abandoned base using 343 Guilty Spark as their prophet.

The gas mine appears in the Halo 2 levels, The Arbiter, and The Oracle.