From Halopedia, the Halo wiki
Menachite Mountain was a mountain in the Highland Mountains on Reach. Housed underneath it was the secret ONI CASTLE Base, built out of a titanium mine, which was in turn placed above a network of volcanic tunnels and further down, a series of Forerunner ruins, which housed a Forerunner crystal. The space under the base was filled with explosives powerful enough to level the whole facility. Oftentimes the Spartan-IIs would train in the tunnels.
Fall of Reach
When the Spartan-II Red Team was deployed to the surface of Reach, Frederic-104, Red Leader, split the Spartans into four groups. Team Delta was assigned to secure Menachite Mountain as a fallback point for Red Team if necessary. Team Delta consisted of all the seriously wounded Spartans and the surviving Marines of Charlie Company, but only some Spartans actually managed to reach the mountain.
As the Covenant surrounded the mountain, the surviving Spartans managed to enter the facility using the secret Oly Oly Oxen Free code, where they encountered Dr. Catherine Halsey, who had stayed behind to clear information and data inside the base should the Covenant gain entrance. Halsey treated the Spartans of their various wounds as the Covenant began to dig in the mountain searching for a Forerunner artifact. The humans only barely escaped with experimental prototype weapons and MJOLNIR Mark V armor upgrades, and as they retreated, they destroyed what was left of the mountain behind them to annihilate CASTLE base.
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