From Halopedia, the Halo wiki
Containment shields are massive hard light barriers that can be created by Sentinel forces to quarantine an area. One was projected from the Sentinel Wall around Installation 05's Library. One of the many containment measures built by the Forerunners, it helped contain and enclose the Flood inside the Quarantine Zone, and keep out anything that could sustain the Flood. It had to be lowered and deactivated in order to enter the Library and recover the Activation Index. It can be also breached with heavy firepower, such as that of a Banished Scarab. A containment shield does not shut down when the defense network is disabled.
During the Battle of Installation 05, the containment shield kept UNSC and Covenant forces stationed on Delta Halo from reaching the Index. In order to gain access to the Library and retrieve the Index, the Hierarchs of the Covenant dispatched Arbiter Thel 'Vadamee to lower the shield. The Arbiter fought his way through the Sentinel defenders to the enormous chamber containing the controls for the barrier. After destroying the Sentinel Enforcer guarding the chamber, the Arbiter retracted four piston-like plug locks which unlocked the main control panel, allowing him to lower the shield. This caused the entire two-tiered control platform to move to the other side of a large chasm in the middle of the Sentinel Wall in a manner similar to an anti-gravity gondola.
The deactivation of the shield allowed access to the Quarantine Zone by the Covenant and the UNSC, which also gave the Flood hundreds of new hosts. The Flood was released onto the rest of Installation 05. As a result of this, the UNSC In Amber Clad was taken over by the Flood and flown into the Covenant holy city of High Charity.
In December 2552, three days after the Human-Covenant War ended, monitor 000 Tragic Solitude had a containment shield set up around the ruins of High Charity to contain the Flood that had survived the detonation of the city's reactors and the firing of Installation 08. As an additional security measure, Tragic Solitude had the Sentinels raze the perimeter and modify the refugia to ensure that no sentient life survived nearby in order to deprive the Flood of any host bodies in close proximity to their prison. Tragic Solitude also kept a battalion of Sentinels on patrol in the vicinity of the containment shield, not only to guard against the Flood, but to guard against future invaders who might try to breach the shield and thus release the Flood.
During the Second Ark Conflict in June 2559, Atriox, the leader of the Banished, ordered Jiralhanae brothers Pavium and Voridus to salvage anything useful from the shipwrecks near High Charity, but to only scout the shell and leave it alone, well-aware of the danger posed by any surviving Flood. However, like many of the Banished, Voridus brushed off the Flood as another Covenant lie and didn't heed Atriox's warnings. Instead, Voridus deactivated the Ark's Sentinel defense network using a terminal near High Charity and burned a hole in the containment shield with a Banished Scarab. When Voridus' troops entered the wreckage, they were swarmed by Flood infection forms which attacked the Banished and escaped onto the Ark through the breach in the containment shield created by Voridus, unimpeded by the Ark's Sentinels due to Voridus deactivating them.
Voridus eventually managed to reactivate the Sentinel defense network and the Banished, with the help of the Sentinels, slew a Proto-Gravemind that the Flood formed before it could become a full-fledged Gravemind. With the Sentinel defense network reactivated, thousands of Sentinels began a cleansing operation, supported by the Banished. The Sentinels and the Banished were able to contain the Flood outbreak and seal off High Charity once again.
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