Installation 00 [Updated]

Like the rest of the newer, more efficient Halo Array, Installation 00 was constructed sometime between 98,445 BCE and 97,445 BCE under orders of Master Builder Faber, to address problems with the greater Ark and its larger Halos. While plans for the second Ark had existed for some time, its construction was delayed by the Old Council to avoid the Builders from gaining too much power. The lesser Ark, as it was known at the time, was designed to produce smaller but much more powerful Halo rings, which could be transported more efficiently and could deal far more widespread damage. Until the very last days of the Forerunner-Flood war, the lesser Ark, its new Halos, and its location were kept a well-guarded secret; only the most top-tier Forerunners knew of its existence, and the only one who knew the exact coordinates of the installation was Faber. Whereas the greater Ark acted as the Forerunners' final line of defense and command center, the lesser Ark would serve the function of a last fallback point where its own array of rings could be swiftly distributed to their designated locations across the galaxy and fired to cleanse the galaxy of all sentient life as a last resort.

In the endgame of the Forerunners' war with the Flood, the Forerunner Contender-class artificial intelligence Mendicant Bias, which had been corrupted by the Gravemind and defected to the Flood, attacked and destroyed the greater Ark, killing the majority of the remaining Forerunners who had taken refuge there. Using coordinates provided by the Master Builder, a handful of survivors, including the IsoDidact, managed to escape to the lesser Ark where they prepared their weapons of last resort. In order to buy the IsoDidact time, the Librarian traveled to Earth, broadcasting a feint transmission intended to draw the Flood's attention to her. She spent her last days in the plains near Mount Kilimanjaro, near the site of the slipspace portal to Installation 00. Mendicant Bias knew of the secret Ark and how to access it through the Keyships, though he did not know of its exact location. In order to devise the best possible defense against Mendicant Bias, the Forerunners deployed Offensive Bias, a military AI formerly in control of the greater Ark and Mendicant's replacement, and sent it to counter Mendicant Bias' fleet in advance before it could reach the Ark. While Offensive Bias engaged Mendicant's fleet, the IsoDidact completed preparations for the Halo Array and fired the rings from the Ark, purging the galaxy of all life.

Following the Array's firing, Offensive Bias defeated Mendicant Bias within minutes. Offensive decided to spare the last portion of its opponent and delivered it to Installation 00 for study. There, the remaining Forerunners, led by the IsoDidact, held a trial for the AI for betraying the ecumene. The Didact decided to keep Mendicant alive as it had intimate knowledge of the Flood and could be called upon in case of their return. Bias was entombed in the installation with only one thought allowed to him: atonement.

Immediately following the Prophet of Regret's botched invasion of Earth on October 20, 2552, the Prophet of Truth dispatched a Jiralhanae-led fleet to continue the assault and to uncover what he believed to be the Ark. By November 17, the Covenant had uncovered the structure they had spent the last month unearthing. Truth, though his Forerunner Dreadnought, activated the device, which was actually just a portal generating structure which led to Installation 00.

Truth led his fleet through the portal; after defeating a Flood invasion of the town of Voi, a joint United Nations Space Command - Sangheili strike force followed the Prophet's forces. Upon their arrival on December 11, the latter force were attacked by the Covenant, which held a 3:1 numerical advantage.Despite this, the Sangheili Fleet of Retribution crushed Truth's forces.

After engaging the Covenant across the installation, Spartan John-117 and Arbiter Thel 'Vadam entered the citadel, where they found Truth, who had been infected by the Flood. Soon after 'Vadam killed the Prophet, effectively destroying the Covenant and ending the war, he, the Spartan, and 343 Guilty Spark, the former monitor of Installation 04, discovered that a replacement Installation 04 was being constructed by the Ark's foundry. The Spartan and the Arbiter soon rescued the AI Cortana, who held Installation 04's activation index, from the Flood-infested Covenant capital city, High Charity. They, with Sergeant Major Avery Johnson, assaulted the Halo's control room to activate the ring, destroying both the Flood and the remaining Covenant forces. They succeeded, though Johnson was killed by the monitor. While Guilty Spark refused to allow his new Halo to be destroyed, he also implied that he was protecting the Ark. The partially constructed installation's activation tore the ring apart and severely damaged the Ark.

In March 2553, the Office of Naval Intelligence deployed an automated probe-carrying spacecraft to the damaged Ark using advanced slipspace technology. The ship arrived in November and dispersed its probes, although all communications with the devices were cut off after initial transmissions confirming the Ark's survival. In response, the UNSC Rubicon was sent from Luna in December 2553 to conduct a more thorough investigation of the installation's ravaged surface. Due to the damage dealt to the Ark's life support systems and its artificial sun, heavily inclement weather became the norm.

The Rubicon carried several remote contact teams, who landed on the damaged Ark and attempted to find the source of the mysterious signal. After about eight days, contact with all teams was lost; only one team, RCT-06, returned, having sustained heavy casualties and carrying with them a device of unknown nature. The armature's origin was subjected to debate, but it was believed by the team to be the source of the beacon picked up by the initial investigative group. The device, actually a broken, barely active monitor, began to relate its past life as a human: Chakas. Though the ship's ONI attaché initially doubted the veracity of the monitor's account, the science team leader pointed out that it was consistent with the "Bornstellar Relation" found on Onyx.

Eventually, the monitor revealed that it was none other than 343 Guilty Spark himself. Claiming that he knew the location of the Librarian, whom he believed to be alive, Spark subsumed the ship's AI, knocked out the crew via the life support systems, and hijacked the ship. Communications with the Rubicon halted within 48 hours of its last report. Search and rescue teams were then deployed to the site, but reported no findings.

The last of its ground team, Bobby Kodiak, was left stranded when the Rubicon left, and mortally wounded by attacking chaefka. It was then that he was contacted by the Ark's monitor, 000 Tragic Solitude, who had survived the Ark's devastation and sought to repair the station and get revenge. Offering him a chance to live, Tragic Solitude rescued Kodiak and mended his body with cybernetics, turning him into a cyborg and putting him under the monitor's control. For the following two years, Solitude then repaired the Ark and prepared for an attack on the galaxy.

In 2555, the Ark was partially restored and Tragic Solitude enacted its plan. Using Kodiak to activate the process, Tragic Solitude initiated a countdown to the rings' firing in several weeks. This was so that someone within the Milky Way galaxy would open a portal to the Ark in an attempt to stop the countdown, allowing the monitor to dispatch its Retriever Sentinels to kill all sentient species and mine their planets to repair the Ark. The countdown was discovered on the intact Halo rings soon after and a human-Sangheili joint task force departed to the Ark through the Portal at Voi. The expedition's ship was intercepted and brought down above the Ark by Tragic Solitude's Retrievers, but the crew survived and managed to reach the Array's control center and stop the countdown.

However, the portal was still open and Tragic Solitude deployed a fleet of Retrievers to Earth in an attempt to mine the Sol system's worlds, causing heavy damage to the region surrounding the portal as well as the UNSC Home Fleet. However, the UNSC-Sangheili crew managed to thwart the monitor's plan and the Retrievers were recalled, with the monitor ending up destroyed. UNSC reinforcements were deployed through the portal to the Ark soon after and rescued the expeditionary crew. With Solitude destroyed and the Retrievers now dormant, the UNSC began to establish research facilities on the Ark and study the Sentinels in hopes that they could be commanded to continue repairing the installation.

Following the Created takeover of human space the Ark's failsafe measures prevented Cortana and her allies from being able to access the installation. In 2559, the Banished, led by the Jiralhanae warlord, Atriox, laid siege on the UNSC bases on Installation 00, among them the Henry Lamb research outpost. Shortly after the Banished's attack, the UNSC Spirit of Fire, drifting in space for 28 years, was abruptly transported to the Ark by a mysterious entity. Those onboard the Spirit of Fire came into conflict with the forces of the Banished on the surface soon after. During this time, there was also a Halo residing in the Ark's center.