Menachite Mountain is a mountain in the Highland Mountains on Reach. Housed underneath it is 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 was housing and hiding the Forerunner Crystal. The space under the base was filled with explosives powerful enough to level the whole facility. The SPARTAN-II's would often train in the tunnels.