In Threshold's atmosphere, there is a Forerunner facility built to mine the gas from the planet. 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.[1]
The Covenant Heretics made their headquarters out of the abandoned base using 343 Guilty Spark as their prophet.
The station was lost when the Arbiter cut the cables holding up the station, dropping it into Threshold's depths. This action was praised by 343 Guilty Spark saying "The cable on top of this mine were designed as fail safes in case an outbreak took place. I am glad someone had the foresight to cut it!"
Appears in the Halo 2 levels, The Arbiter, and The Oracle.