Alpha Base (Installation 04)

Alpha Base was the main tactical Firebase used by the remaining UNSC forces after planetfall on Installation 04. It was located on a small mesa several kilometers down-spin from the crash site of the 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.

The humans set up camp around the base using the supplies they had scrounged from the Autumn and their Human Entry Vehicles. They lived in Cubicles and ate mostly Meals Ready to Eat. A good deal of the activities also took place in caverns underneath the facility, in the Forerunner structures.

History
The Covenant occupied this mesa when they discovered the ring, using mostly a light compliment of Grunts, Jackals, and Elites, as well as a few Banshees. It provided a good view of the surrounding areas, and the steep embankments around it made it hard to reach from the ground. Strategically placed Shade turrets made the mesa hard to assault from the air as well.

When the Pillar of Autumn was attacked above the Halo, the ODSTs under command of Major Antonio Silva abandon the ship aboard Human Entry Vehicles and most of them landed around the base. Silva saw the tactical advantage to the butte, and orders First Lieutenant Melissa McKay to take it.

The ODSTs are successful in taking the base, but they are undersupplied. Silva then orders McKay on a convoy to the Pillar of Autumn crash site, and she successfully gathers materials and resupplies the base. It then becomes the central base of the survivors of the Autumn, as all known surviving naval personnel and Pelican ships are relocated there.

Later, the base is fortified with Shade Turrets, salvaged 50mm cannons from the wrecked Autumn, and sensor equipment operated by the AI Wellsley. These came in handy when Covenant forces led by Zuka 'Zamamee and Noga 'Putumee attack the base in hopes of finding the Master Chief. The attack fails, and the humans hold the mesa.

It is later discovered that the Flood have been attempting to break into the base and are only impeded by a single metal grate leading to an unnoticed group of caverns. The entrances to these caverns are successfully sealed, and the Flood are delayed from overtaking the base. By the time they manage to break through, the UNSC forces have abandoned the base in hopes of leaving the installation aboard the Truth and Reconciliation.