Threshold gas mine

Were you looking for the Battle of the Gas Mine? In Threshold's atmosphere, there is a Forerunner facility built to mine 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 it has 3 tubes extracting gas from threshold (although only two worked on the game) the station is divided into 5 modules: 2 Gas Refinement arms (each one with its own cargo hanger), 2 Laboratory modules for the research of the Flood, and the Final module is the control section of the mine. Each of these modules can be accessed by catwalks or landing sites (there are no other visible means for access). The main cable is held in place by three equidistant holds on the top level; supposedly the holds could be released by the main control of the station.

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, after a large battle with the Heretics and the Flood, cut the cables holding up the station, dropping it into Threshold's depths.

The Covenant Heretics made their headquarters out of the abandoned base after being taught the true functions of the Halo by 343 Guilty Spark. The gas mine appears in the Halo 2 levels, The Arbiter, and The Oracle.