Arcadia ONI facility
From Halopedia, the Halo wiki
The base had been built onto a plateau, likely called "Beasley's Plateau", its edge overlooking the valley that contains the Forerunner ruins. The landing area consisted of five landing platforms, all of which were connected to a large central field. This section had two reactors powering it. It also had a hangar and several other buildings around it, as well as tunnels that led to other sections of the base. There were also multiple air control towers around the landing area. The base also had an on-site Landing pad and air tower near to the site of the Firebase deployed in the area, allowing for the requisition of M145D Rhino artillery.
In addition to the landing area, the base extended further along the plateau, having several more installations, mostly roads, fields and air control towers as well as large bunker-like structures. Several installations were also located in the outskirts of Pirth City, and were largely destroyed in the Covenant attack. Several ONI reactors were present in Pirth City, and were utilised by Spirit of Fire forces during the battle.
The base's placement near the Forerunner ruins may have been the result of a coincidence, or it may have been originally constructed to serve as a site to conduct research on possible Forerunner findings, and to keep the planet's civilian populace away from the area.
During the Battle for Arcadia in 2531, the base was apparently attacked by the Covenant during the opening stages of the engagement, as much of the base was left in ruins when UNSC reinforcements from the UNSC Spirit of Fire arrived. The UNSC used the landing area of the apparently-deserted base as their base of operations, when they attacked a Covenant base inside a dome-shaped energy shield. They used an air pad on the base to call in Rhino tanks from the UNSC Pillar of Autumn in orbit, and placed the tanks on the landing pads on the edge of the cliff. After a few unsuccessful attempts, they managed to destroy the Covenant outposts in the area and breach the energy field. After destroying it, they proceeded down to the valley to investigate the area.
The remnants of the base were likely destroyed when the Covenant returned and destroyed Arcadia in 2549.
The Halo Wars campaign level Dome of Light is set in the landing area of the base, while the multiplayer maps Beasley's Plateau and Pirth Outskirts take place in and around sections located deeper inside the jungle. Pirth Outskirts features many of the same kinds of Forerunner ruins as featured in the mission Scarab, while Beasley's Plateau is centered on the Forerunner lifesupport pod.
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