Threshold gas mine
From Halopedia, the Halo wiki
Summary
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.
Appears in the Halo 2 levels, The Arbiter, and The Oracle.
