The Ark is a Forerunner construct beyond the edges of charted space. The only known way to reach it is through a portal, like the (now inoperable), one on Earth. The Ark's purpose is twofold: it acts a refuge from the Halo installations, as it is beyond their operational range, and it also acts as a control center for the Halos. The Ark also serves as a factory for Halo rings, and possibly Shield Worlds. The latter half of Halo 3 takes part on this construct.
The Ark was constructed, along with the Halo installations, during the 300 year war the Forerunner fought with the Flood. The Forerunners constructed the Ark in secret, out of reach of the Flood. The Ark was their refuge against the parasite that was assaulting them. While the Forerunner fleet fought the Flood in system after system, they were inevitably crushed by superior numbers, and the Forerunner began to see that activating the Halos was the only course of action they had left. The Forerunner began evacuation of civilians to the Ark and Shield Worlds such as Onyx, while their military remained to clash with The Flood. However, unwilling to doom many of the worlds they could not evacuate, the Forerunners stalled in activating the Halo arrays, and instead searched for an alternative method to defeat the Flood.
Their answer was Mendicant Bias, the most advanced Artificial Intelligence they had ever created. Mendicant Bias was to take a group of Forerunner ships deep into Flood-controlled space and attack the central Gravemind. A strike on this central intelligence would halt the Flood's outward advance, and buy the Forerunner's more time in their war. Mendicant Bias succeeded in locating a Gravemind, but the Gravemind initiated communications, and convinced Mendicant Bias that the Flood was the next evolutionary step, and the Forerunners needed to be absorbed to continue along the evolutionary path. Mendicant Bias turned on it's makers, and revealed the location of the Ark to the Flood. After confirming Mendicant Bias as rampant (See: Marathon
), the Forerunners knew they had no choice left. With Mendicant Bias on his way to the Ark with the Flood, the Forerunner constructed one last AI, Offensive Bias, to defeat him, and prepared to fire the rings. Offensive Bias was created purely for combat, and had no capacity to side with the Flood as Mendicant had. The two met in combat, each controlling a massive fleet. Before Mendicant Bias could overwhelm Offensive with sheer numbers, the Forerunners back on the Ark activated the Halos, and all crewmen on both sides, along with all beings that had not been either transported to the Ark or a Shield World, perished, evening the playing field and allowing Offensive Bias to defeat Mendicant Bias and his fleet. Offensive took what remained of Mendicant Bias and returned it to the Ark for study. What happened to the Forerunners on the Ark after they defeated the Flood is unknown. However, Mendicant Bias remained on the Ark, and a hidden message from him can be seen in a Terminal on the last level of Halo 3 if you are playing on Legendary.
Almost 100,000 years later, after The Covenant
attempt to activate Delta Halo was thwarted, both the UNSC and the Covenant learned of the existence of the Ark, and it's ability to activate all the Halo rings at once. The Prophet of Truth and his Brutes headed to Earth, and immediately unearthed a portal that led to the Ark. The Elites and one UNSC ship followed, attempting to stop Truth from firing the rings. The combined forces broke through the barriers Truth's forces had erected around the Ark's control room, but the Flood infested High Charity jumped into the sky above the Ark as they were beginning their final assault on the control room. The Flood temporarily allied themselves with the Master Chief
and the Arbiter
as they fought through the last waves of Truth's forces, because activation of the rings posed a threat to all of them. However, after the Arbiter killed the Prophet of Truth, the Flood turned on the Chief and the Arbiter, attempting to kill them. The two escape, and discover a partially rebuilt Alpha Halo that was being built on the Ark. The Chief retrieves Cortana, who has the activation index from the original
Alpha Halo, and the head up to the Halo, which is hovering in the Sky above the Ark.They succeed in activating the ring, but because it is not complete, the firing sequence destroys the rebuilt ring, and seriously damages the Ark. Following these events, the only known portal to the Ark closes. It is likely that the Ark will repair itself, and perhaps eventually even build another
Alpha Halo, as an army of sentinels is stationed on the construct.
The installation is a massive disk with eight curved arms. Like the Halo rings, the entire surface is terra-formed and includes locations like forests and deserts. The ark is extremely large, large enough to support a small planet in its void. This void conceals the Halo foundry, which is where the Halo replacement rings are constructed. At a distance of 2^18th light years from the galactic center it is effectively outside the galaxy. As such, the Milky Way galaxy can actually be seen when your character
lands on the ark for the first time. In fact, an ODST soldier actually notes it with shock. This removal from known space was intentional, so that the Halo rings could be fired without killing any of the Forerunners or other species that were on the Ark. Because the Halo rings only were effective within a certain operational range, they could be used against the Flood without damaging anyone on the Ark. Spartan-117 used this geographical isolation to his advantage, when he activated the rebuilt Installation 04 above the Ark, destroying the Flood without harming any other sentient species in the galaxy.