The Library is a huge structure that is mostly located in the center of Installations and Forerunner planets. It was first featured in Halo: Combat Evolved in the The Library level, when the monitor of Installation 04 teleported the Master Chief to its inner structure.
The library is designed by the Forerunners to store data and informations about researches regarding the Flood, the data itself is located in the Index Chamber which holds the Index. The Index is also a key that is capable to activate a sequence via the Control Room and destroy all life forms in nearby perimeter, which also triggers other Installations and destroy the whole galaxy. By doing so, the Floods will lose their prey and starve to death and extinction.The structure of the Libraries are admirably large and symmetric, vast hallways and massive elevators were diversely scattered alongside the hundred of vents which the Monitor and the Sentinel travel through. It is also designed to hold back Flood attacks, but that was not applied in Installation 04. The Library failed to contain the Flood outbreak, they were in every section of it except for the Index Chamber.
 Installation 04's Library
In the first video game of the Halo series, Halo: Combat Evolved, the Master Chief was teleported by 343 Guilty Spark, the Installation 04 Monitor, to the inner structure of the Library. The Monitor asked the Master Chief to aid him in containing the Flood outbreak by retrieving the Index. As the Master Chief travels through the hallways of the Library, enormous amount of Floods were engaging and spreading. The Sentinels seemed outnumbered and weren't witnessed occasionally.
The Master Chief made it to the Index Chamber and retrieved the Index, which in turn results in the destruction of this Installation by activating the fuel rods in the Pillar of Autumn. After its destruction, another replacement was built, the Installation 04B, with a newer and more efficient Library.
 Installation 05's Library
The Library of Installation 05 witnessed a larger Flood outbreak than the previous one on Installation 04, the Flood spread quickly and damaged a lot of facilities and defenses. When the UNSC Marines including Miranda Keyes arrived to retrieve the Index, they were surprised at the Flood's power and number, most of the marines were infected and turned to Flood forms.
In Halo 2, the Arbiter went to Installation 05 and into its Library via an alternative route using the Gondola due to the assistance that the Monitor didn't give. The Arbiter went to retrieve the index only to find Tartarus stealing it, and ditching him to bane.
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