Installation 00

Installation 00, also known as The Ark, is an enigmatic Forerunner installation from which all the Halo Installations across the galaxy can be activated simultaneously. In addition, it functions as a safe haven from the Halo network's strategic pulse and as a facility to construct replacement Halos. It was first mentioned by 343 Guilty Spark at the conclusion of Halo 2, where he explains the fail-safe protocol of Installation 05 after its unexpected shutdown. The Ark is about 218 (exactly 262,144) light years from  the galactic center.

Layout and Function
00 resembles a massive disk with a series of eight curved arms, similar to an octopus. It's core resembles that of a very old (crumbling) planet. This is possibly where the Forerunners found a planet able to sustain human life outside of the galaxy out of the Flood's reach. Then they started constructing the Foundry around it and later the Ark. The entire upper surface is terra-formed, with oceans, forests and even a large desert, where the Master Chief first makes landfall. The structure is the largest Forerunner construct seen in the series, possibly a hyper-structure knowing that fortress worlds are 10,000 km in diameter approximately. It is theorized to be an Alderson Disk. A simple inspection of the holographic representation reveals it to be anywhere from 90,000 to 100,000 kilometers (55,890 to 62,100 miles) in diameter, making it the largest Forerunner construct known to this day.

The Ark is large enough to support a small planetoid in its center, which is surrounded by a void. This void conceals the Halo Foundry, which constructs rings and sends them to their orbital destinations. Most interestingly, the Ark is not actually in the Milky Way, and the galaxy can actually be seen in the sky, as noted by a stunned ODST soldier at the start of the level The Ark. As such, it's outside the believed range of any Halo installation. The Ark also has an artificial sun floating above it. An observer can see this most clearly on the level Halo when looking up at it. Much of its massive size and the core in the middle can be seen during the Warthog run on Installation 04 (II)

The two shown primary functions of Installation 00 are as a remote for activating the entire Halo network with safety of being out of the halos range, and as an automated construction facility for the creation of replacement Halo installations (ensuring that the network continues to operate even if any rings are destroyed). The construction facility is in the Ark's Core, concealed by clouds.

After the level Floodgate, the entire remainder of Halo 3 takes place on or around Installation 00. While there, a number of conflicts including the UNSC and Covenant Separatists, the Covenant Loyalists and Flood take place. Cortana claims that the premature firing of the second Installation 04 by the Master Chief "did a number on the Ark,". It was heavily damaged, but no one knows what (if anything) has survived on the surface.

Atmosphere
Along with all Forerunner constructs, the Ark consists of an atmosphere similar to earth, comprised of oxygen and hydrogen. This makes all Human-Covenant war species able to live on this planet (excluding Grunts who depend on methane tanks to sustain life). Also, weather radically changes while being on the ark (example there was desert in The Ark and it was Snowy in The Covenant.

History
Installation 00 is a part of the Forerunner construct designed to wipe the Flood from the Galaxy. Located 2 18 (about 262,144) light-years outside the Milky Way galaxy, Installation 00 is safe from the Halo effect (as it is referred to by the Forerunners in the terminal transcripts) that kills all sentient lifeforms; the main Forerunner behind its use was Didact. The purpose of the installation is to remotely activate every halo ring, from outside the blast radius. This is a fail safe system that is only activated by an aborted firing such as the one that occurred at the end of Halo 2. In addition, Installation 00 can also function as an automated factory to construct replacement Halo installations, thereby insuring that the Halo array is always ready to fire.

"Every vessel we can fill, we send to the Ark. I dare not cease the mission. Not now, not until I've done all I can. Each one of these souls is finite and precious."

- --Transcript of a message from the Librarian to Didact, interpreted post-cataclysm and retrieved from the first Terminal on the level "The Ark"

Apparently, while the rest of the Forerunners attempted to defeat the Flood through any military way possible, a plan B was also approved to be put into the works, which included the construction of the Halos, Shield Worlds like Onyx, and The Ark.

In this final resort plan, the Librarian was to index every sentient species in the Galaxy, and bring some to the Ark, and then presumably some Forerunners, and these last sentient species would hide in the Ark's secret location, until the Flood starved and they could go back to the Milky Way.

However, things did not go as planned, as the Forerunner military AI Mendicant Bias turned on the Forerunners, and began assisting the Flood. Mendicant Bias knew of the Ark, and how to access it through the Key Ships, but did not know of its exact location. The Forerunners then constructed another military AI, Offensive Bias, in order to devise the best possible defense against Mendicant Bias and his fleet. The Librarian detonates all of the Key Ships in his/her command, and spends his/her last days upon a mountain (possibly Mt. Kilimanjaro) near the site of the portal to the Ark, for aesthetic reasons.

Portal
The Portal is a Forerunner creation which is designed to open a Slipspace rift to enable access to Installation 00. It is located near the cities of New Mombasa and Voi in Africa.

As first seen in the Halo 3 Announcement Trailer and numerous other promotional materials, this construct was originally thought to be the Ark itself; however, this was disproved with the release of Halo 3. The Portal was partly uncovered when Regret's carrier destroyed New Mombasa in Halo 2, and completely excavated by Covenant Loyalist forces later on.

In Halo 2, the Prophet of Regret lands his carrier above New Mombasa and deploys a large ground army. UNSC Marines are subsequently able to beat the Covenant to a retreating standstill, apparently causing him to attempt a dangerous atmospheric Slipspace transition. This maneuver obliterates New Mombasa and forms a deep crater, partially revealing the Portal.

By the time of Halo 3, Covenant Loyalists begin excavating the portal from the crater and are able to completely uncover it. The Prophet of Truth activates it at the conclusion of the level The Storm. It forms a massive portal, which the Forerunner ship immediately enters (presumably carrying Truth), followed immediately by the Covenant Loyalist ships. At the end of the level Floodgate human and Covenant Separatists forces resolve to follow Truth's ships through the portal, and end up at Installation 00. At the end of the game, the damage caused to the Installation causes the portal to collapse, severing the Dawn in half as it passes back through. The forward section (carrying the Arbiter) makes it back to Earth (plunging into an unspecified body of water) and is rescued soon by human workers, but the aft section (holding the Master Chief and Cortana) does not. The full, true nature of Halo's effect is still not understood, especially since the Chief survived its blast. The civilians and military on Earth speculate that they are dead, and Cortana has sent a beacon to Earth. Master Chief then goes into a "deep sleep"(also known as cryo sleep), since the beacon may take months, even years to reach Earth.

Apparently, the portal requires a Forerunner Ship to function. Originally, this ship was docked in High Charity, and is seen jumping to Earth in the final cutscene of Halo 2. As shown in Halo 3, the Forerunner Dreadnought was docked in the middle of the structure, and gave off a pulsing beam of light that helped power the main beam, causing the blinding light shown in the trailer.

The Foundry
The Foundry is the open area at the heart of Installation 00, and is the automated factory for replacement Halo installations. Raw materials for the process are supplied by an Earth-sized planetoid in the center. The Core is large enough to make several replacement Halo's, as seen in Halo 3 when it produced a replacement ring for Installation 04 of Halo: Combat Evolved.

Guilty Spark meets Ark AI
(04-343 is reference to Guilty Spark. 343 being his number designation, 04 being the Installation he is assigned to.)

This Transcript is taken from the third Terminal which is found in the game. It basically tells that 343 Guilty Spark is being warned for accessing Installation 00's systems and is warned that even though he is a Monitor of an Installation he has no right to be accessing systems on the Ark. He also learns that Installation 00 has the capability of making another Halo Installation and manages to stop the AI from sending Sentinels after him. Even though Guilty Spark found out that Installation 00 has the capability of making another Halo, he acted like he did not know this in the level of Halo 3 called "The Covenant" when the Installation 04 was being rebuilt. Terminal #3, datalog:

'''Warning: Your intrusion has been logged.

'''04-343 (errant): Excuse me?

'''Your intrusion has been logged. And now it has been halted.

'''04-343(errant): On whose authority?

'''Advice: Any further attempt to access [insects under stone] will result in your immediate addition to local Sentinels' targeting ledger.

'''04-343 (errant): Vexation! I am the Monitor of --

'''Judgment: Your authority means nothing here.

'''04-343 (errant): Impatience!

'''04-343 (errant): I have told you who I am. Who are you?

'''All our makers once held dear. [Alexandria before the Fire]

'''04-343 (errant): Sincere apology. But how --

'''Explanation: This facility is host to the [Librarians'] final --

'''04-343 (errant): The archive is intact?! Then our makers' plan --

'''But also maintains [bellows, crucible, castings]

'''04-343 (errant): A what?

'''[bellows, crucible] --

'''<font color="#0000cc">04-343 (errant): A Foundry?

'''<font color="#0000cc">04-343 (errant): For what purpose?!

'''<font color="#ff3333">Warning: Your intrusion has been logged.

'''<font color="#ff3333">Advise: Any further attempt to access will result --

'''<font color="#0000cc">04-343 (errant): Indignant!

'''<font color="#ff3333">-- immediate addition to local sentinels' targeting ledger. '''

(terminal gets overridden)

A lot of commands show up, including this somewhat creepy message said by Mendicant Bias:

I. SEE. YOU. RECLAIMER.

Trivia

 * At the end of Halo 2, the Forerunner Dreadnought, carrying the Prophet of Truth, jumped into Slipspace from near High Charity and Delta Halo, heading for Earth. By Halo 3, it had landed in the center of the completely uncovered Ark Portal. The reasons for this landing is that the Forerunner ship acts as the "activator" for the Ark portal by being lowered into the ground and activating the pylons that discharge the beams.
 * Within the Halo Graphic Novel, in the short comic Second Sunrise Over New Mombasa, a transmission is intercepted between the Covenant assault forces stating that once the humans were dealt with in the First Battle of Earth, they would start uncovering the Ark.
 * There is also, of course, the strongly supported theory that human life originated in East Africa, usually said as the Great Rift Valley. New Mombasa is in this area, so is the Ark Portal.
 * In the fifth server of the Iris VMC it was stated that the Ark was built to protect humanity from the Flood and the activation of the Halos.
 * The real world, Ark of the Covenant which according to the old Testament contains the stone tablets of the ten commandments, is the holiest object of the Jewish faith. The Ark of the Covenant is popularly believed to lie somewhere in East Africa, albeit in Ethiopia rather than Kenya.
 * The "Ark" seen in the announcement trailer is structurally identical to the Forerunner sanctuary described in the final chapters of Ghosts of Onyx (albeit on a much grander scale), complete with thirteen metal fins and a brilliant, blue-white portal in the ground. However, it is shown without the thirteen concentric rings rising around the portal, which means that it was either built without them or they will raise as the structure opens.
 * During the level The Ark, 343 Guilty Spark mentions that the Ark is 218 (262,144) light years away from the galactic center. As the Milky Way is around 100,000-120,000 light years in diameter, the installation is beyond the reaches of galactic space, thereby keeping anyone who is on it safe from the firing of the Halos.
 * The "core" of the Ark, as it was called by 343 Guilty Spark, is a planetoid that serves as the Foundry mentioned in the aforementioned conversation between 343 Guilty Spark and the Ark's embedded AI. Investigation from afar reveals that the surface of the core has been strip mined away showing a glowing center beneath the crust. The planetoid was mined in order to provide raw materials for the creation of new Halo Installations. Thunderstorms can be seen from time to time raging across the planet's surface
 * The entire core of the Ark is protected by a large energy shield. Part of this shield was disabled in The Covenant to gain access to the Citadel.
 * A member of HBO estimated the size of the Ark by comparing its size to a Halo installation and Earth and estimated its size to be approximately 90,000 to 100,000 kilometers in diameter. See link here.
 * At one point in the level The Ark, 343 Guilty Spark mentions that he assumed that the Ark was a "shield installation" but he concluded that he must have been mistaken. This could have been a reference to the Shield World.
 * In the Citadel after deactivating the Ark you can see the specific environments of the other Installations.
 * The Ark may have further Biblical connections such as Noah's Ark because he uses the Ark to escape the Flood, taking examples of each species with him to save them, as the Forerunners did.
 * After the Dreadnought reaches the Ark, Mendicant Bias transfers into the Ark, explaining why he can "see" Master Chief through the Terminals in gameplay.

Links

 * Halo.Bungie.Org's Ark forum.
 * Alderson Disk, what the Ark is theorized to be.