Quarantine zone (location)

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The Quarantine Zone was a perimeter surrounding the Library of Installation 05, located within the Sentinel Wall. The Sentinel Wall, along with the containment shield it projected, had kept much of the Flood from spreading during the eons-long outbreak on Delta Halo, allowing the Sentinels to contain the parasite, but the shield's deactivation by Arbiter Thel 'Vadamee during his search for the Activation Index allowed the Flood to move freely about the ring.

Overview
The area was guarded and patrolled by various Forerunner constructs, ranging from the small but necessary Constructors, to the standard Sentinels and Sentinel Majors, to the titanic Enforcers. The Quarantine Zone was apparently kept starved of all life by the Sentinels, of which more were produced by local manufacturing plants. The only signs of past life, besides the Flood, were dead trees and it was constantly snowing. It seems that the Sentinels or some other systems have caused this climate change in order to prevent life from growing and/or existing in this area, therefore denying the Flood of any possible sources of energy. It is unlikely that the change in climate was enacted by 2401 Penitent Tangent as the Monitor had not sent the requested "security detail" after approximately 287 years since the initial request for one. Likewise, the presence of trees at all indicates that it was not always such a cold climate, and is instead a response to the Flood outbreak.

History
Upon arrival within the Quarantine Zone, Covenant found a four-way fight, against the Flood, Sentinels, and UNSC forces, but after the Arbiter Thel 'Vadamee lowered the shield, more and more Flood had begun to spread

Since vehicles were introduced to the area, the Flood began to operate these machines and use them against the Sentinels and the Covenant. Most of the vehicles were Ghosts, Warthogs, a few Wraiths and a few Scorpion Tanks, all brought into the area by the Covenant and UNSC during their advances towards the Library. With access to heavy military vehicles, the Flood became far more mobile and much more dangerous.

Since the human incursion into the area, many of their Marines were infected by infection forms, converted into combat and carrier forms, furthering the Flood life cycle and allowing them to spread quicker. They also allowed the Flood access to a more diverse array of weapons, including Battle Rifles, SMGs, Shotguns, and more, furthering their capabilities in combat.