Threshold gas mine

"This mining facility predates Installation Alpha by several hundred years. It was retrofitted to research possible offensive and defensive measures against the Flood. Indeed, I designed and oversaw the construction of this facility's various outbreak management systems."

- 343 Guilty Spark

The Gas Mine was a large structure suspended in the atmosphere of the Gas Giant Threshold, the planet around which Installation 04 orbited. It was an expansive automated facility that was created and used by the Forerunners and later, the Heretics. It was destroyed when it fell into Threshold and was crushed by the planet's gravity.

Features
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 are operating during 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. The inside of the gas mine is a curious mix of forerunner and factory, with dozens of moving energy conveyor belts carrying gas canisters about. There is a large gas storage hold, which is accessed by a large circular lift. This hold contains hundreds of canisters, most of which contain Flood, which attack the Arbiter when he uses the lift.

History
The Gas Mine was created by the Forerunners with the sole purpose of mining gas from the atmosphere of Threshold. It operated on a completely automated basis for "untold centuries" with only a handful of Sentinels to maintain the facility. It is unclear why the gas was needed, but the gas canisters found in the Gas mine could be found on the Halos, thus explaining why the installations were built around gas giants.

However, after the Flood was discovered, the Forerunner retrofitted several areas of the gas mine into laboratories to study Flood specimens. 343 Guilty Spark, the future Monitor of Installation 04, personally oversaw some of these modifications. A few hundred years passed, however, with no successful defense against the Flood. Thus, the Forerunners constructed the Halo Rings.

The Gas Mine was unaffected by the first activation of the rings and its contained flood remained intact. It remained untouched for over 100,000 years until the year 2552.

Battle of the Gas Mine
That year, when the Covenant discovered Installation 04, they sent the Fleet of Particular Justice to scour the ring and the system. An Artifact Retrieval team led by Sesa 'Refumee, landed on the Gas Mine attempting to retrieve any useful Forerunner technologies they could find; however, at the conclusion of the Battle of Installation 04, the group found itself marooned on the mine.

While there, the group encountered 343 Guilty Spark, drifting in space from the remains of Installation 04. Listening to his words and finding out the truth about the Halos and the Great Journey, Sesa 'Refumee led a rebellion, his Elites and Grunts shed their former armor for the colors of the Elite homeworld and began broadcasting their beliefs into space, attracting the attention of other Covenant personnel, including the High Prophets. Dubbing his movement "Heresy", the High Prophets ordered Special Operations forces led by Rtas 'Vadumee and the Arbiter to attack the gas mine and end 'Refumee's movement forever. The surprise attack went well; the SpecOps forces were able to infiltrate the facility and pursue the Heretic Leader, even destroying the Heretic Air Force along the way. But complications arose when the Flood were released from their restraints by the Heretics, and overran the entire facility. The Flood multiplied quickly, infecting and killing most of the Heretic forces, making it difficult for the SpecOps Elites to secure the facility. The Heretic leader locked himself in the inner section of the mine, safe from both the Flood and an approaching maelstrom, but the Arbiter responded bluntly by cutting the cables holding the mine up, sending it in a stoic free-fall. This forced the Heretic Leader to escape his hideout with an attempt to evacuate aboard a Covenant Seraph fighter, but the Arbiter caught up to him and they had a intense battle where he was killed. Shortly thereafter, Tartarus appeared in a Phantom, and he, the Arbiter, the SpecOps Elites, and 343 Guilty Spark escaped the mine before it fell into Threshold's inner atmosphere, where it was subsequently destroyed.

Trivia

 * This was Halo 2's first mission location as the Arbiter (and the first place that the player can play as the Arbiter.)
 * It is possible that there are more Gas Mines near other Installations or on Threshold itself.
 * According to 343 Guilty Spark, the cables supporting the Gas Mine, which the Arbiter cut by an Energy Sword, is actually a fail-safe function to prevent the Flood from escaping the mine, should an outbreak occur.
 * The Sentinels found around the Mine are thought to be controlled by Guilty Spark, based on the fact that he is the only Monitor AI seen there, and by hearing a conversation between two Heretic Sangheili. It can be assumed these Sentinels are in fact his, since he oversaw the facility while it was being fitted to study Flood.