The Lesser Ark, more generally known as just the Ark or Installation 00, is a Forerunner construct, designed to both create and control the Neoteric Halo array from outside the Milky Way galaxy. In addition to acting as the central control hub of all seven Halos, the Ark functions as a home for many species from many planets that were protected in order to repopulate the galaxy after the firing of the Halo array. These refugia, as they are known, are physically separated by large walls used to control both the local climates and the migration of species between regions. At the end of the Forerunner-Flood War in 97,445 BCE, the Lesser Ark was used for its intended purpose, to eradicate the galaxy of all sentient life and starve the Flood in what is known as the Great Purification, the only time the Halo Array was fired.
In 2552, the portal to the Ark from Earth was discovered and opened, leading to a battle on the installation to try and stop the Prophet of Truth from activating the entire Halo array. The UNSC and their allies managed to stop Truth, but not before causing significant damage from battles and the firing of Installation 08 overhead of the Ark.
In 2554, the Office of Naval Intelligence sent a team to investigate the status of the Ark, but contact with them was soon lost. Less than a year later in 2555 another team consisting of humans and Sangheili were sent through the portal to the Ark after researchers discovered a countdown that was occurring on all Halos. While there the team discovered the monitor of the Ark, 000 Tragic Solitude, had set the countdown to get the portal opened and send monitors to Earth to harvest it for raw materials. This attack on Earth from the Ark was stopped, and the Ark remained partially repaired for the next several years.
For the next few years, the UNSC and ONI established a research base on the Ark, ensuring the Ark remained stable and learning what they could from the installation. At the onset of the Rule of the Created in 2558, the portal was closed, sealing off the research team from Earth. A month later, the Ark was visited by the Banished, a rogue group of pirates led by the Jiralhanae Atriox, and the research team was killed. In March of 2559, the Spirit of Fire arrived above Installation 00, and began a conflict with the Banished, resulting in the generation of another Halo and the temporary release of the Flood from the wreckage of High Charity.