Alpha Base was the main tactical Firebase used by the remaining UNSC forces after planet fall on Installation 04. It was located on a small mesa several kilometers down-spin from the crash site of the UNSC Pillar of Autumn. The mesa itself housed a collection of Forerunner structures as well as a system of caves stretching beneath the facility and linking with the larger subterranean system of the ring. A.I Wellsley tagged the base as HS2604.