Threshold Mine (Earth-5875)

"This mining facility predates Installation 04 by several hundred years. It was retrofitted to research possible offensive and defensive measures against the xenomorphs. Indeed, I designed and oversaw the construction of this facility's various outbreak management systems."

- - 343 Guilty Spark

The Threshold Mine was a massive forerunner installation suspended in Threshold's atmosphere, the gas giant formerly orbited by Installation 04. It was designed by the forerunners to mine the properties of the gas giant, and was later used by Sesa 'Refumee and his heretic group, the Light of Truth. It was destroyed by the Covenant during a mission against the Light of Truth, as the installation was crushed by Threshold's gravity.

History
The Threshold Mine was designed by the forerunners for the automated mining of hydrogen, helium, and methane from the gas giant Threshold in the Soell system. It featured its own contingent of Sentinels that maintained the facility for the coming years.

Following the discovery of the xenomorphs and the eventual war that started with it, the forerunners retrofitted the station with many laboratories designed to contain and study xenomorph specimens, with 343 Guilty Spark, the Monitor of Installation 04, personally overseeing the construction of the installation. After the xenomorphs proved unstoppable and their infection incurable, the forerunners activated the Halo Array and were extinguished. The mine was not affected by the rings' pulse, and as a result, neither did its captive Ovomorphs, which remained untouched for the next 100,000 years.