Library
From Halopedia, the Halo wiki
Summary
The Library is where the Index is housed. It is kept safe from the Flood by having the only access be a gondola from a Forerunner base, which is protected by an energy shield and a large wall filled with Sentinels and Enforcers. Then, beyond the wall is an artifical lake. It should be noted that one may access the Library though the Halo's teleportation grid.
Halo: Combat Evolved and Halo: The Flood
343 Guilty Spark teleports the Master Chief from the swamp that was part of the previous level, 343 Guilty Spark he was in into the Library of Installation 04. Equipped initially with a Pistol and an Assault Rifle, he must battle through four Flood-filled levels to reach the Index chamber.
It is in this level that 343 makes reference to the Forerunners' technological superiority. He wonders why the Chief had brought "such ineffective weapons" to combat the Flood, and recommends that the Chief upgrades from the MJOLNIR armour, which, according to some Forerunner ranking system, the criteria of which is not gone into, is only Class 2, to a Class 12 combat skin, which he says is necessary to combat the Flood.
Somewhere in the 3rd or 4th section of the Library, the Chief encounters the body of a Marine who was the most recent failed Reclaimer. In The Flood, this Marine is said to have had his body so mutilated that the Flood could not possibly take over him, and he is identified to be Staff Sergeant Marvin Martubo.
Halo 2
In Halo 2, while the Chief goes after Regret, Commander Miranda Keyes, Sergeant Johnson and most of the marines go into the Library to find the Index.
Therefore, the Arbiter, instead of the Chief, is the playable character that has to travel the Library of Delta Halo. This journey encompasses the levels "Sacred Icon" and "Quarantine Zone".
Dropped in the wall from Tartarus' Phantom, the character must go through the wall's area, down pistons and fighting Sentinels to find a gondola. By removing the containment plugs, the Containment Shield guarding the Quarantine Zone can be lowered, enabling the Arbiter to enter the Library proper.
After battling his way through corridors filled with Flood in all three varieties, as well as Sentinels and Enforcers, the Arbiter finds an open-air area where he meets with some air-dropped Elite comrades and reunites with Half-Jaw, the Spec Ops Commander. He now has to defend the Elite encampment from waves of Flood Combat Forms.
After the waves of Flood have ceased their siege, the player, in the guise of the Arbiter, is given the option of choosing either a Ghost or a Spectre to go through the Quarantine Zone with. While battling through the Quarantine Zone, other vehicles, including Warthogs, Wraiths, and a Scorpion can be found. It is first learnt here that Flood Combat Forms now know how to drive vehicles. After going through an open-air section, a mine-like area, and another open-air section, the Arbiter reaches yet another gondola, where Half-Jaw leaves to take care of an approaching group of Flood. Tartarus calls Half-Jaw a coward from the safety of his Phantom and sends a few Elites to aid the Arbiter. The Arbiter takes the gondola into the depths of the Library to find the Index chamber. On the way, he must fend off assaults by Combat Form Flood wielding a variety of weapons. When he finds the Index, he tries to take it from UNSC forces, but he is betrayed by Tartarus and he lands in the throne room of the Gravemind.
