Gao, known to the Forerunners as Edod, is a remote human Outer Colony in the Cordoba system that survived the Human-Covenant War. While nominally under the administration of the Unified Earth Government, both the people and the colonial government of Gao maintain a strong sense of national identity and anti-UEG sentiments are commonplace. The planet's Montero region was home to a vast cavern system — in fact an artificial Forerunner construct designed to support the Maginot Line.
During the Insurrection, Gao openly supported the Insurrectionists. Following the Human-Covenant War, Gao remained highly independence-minded and sought to strengthen its ties with nearby colonies, such as Venezia, which was only nine light years away rather than the UEG.
In May 2553, the UNSC Marine Corps 717th Xeno-Materials Exploitation Battalion protected by Blue Team arrived at the planet to recover a Forerunner ancilla in the Montero Cave System. The UNSC subsequently declared the cave system off-limits to outsiders, but civilians continued to sneak into the caves anyway in search of rumored miracle cures; soon after, an increasing number of these civilians were found brutally murdered. With tensions heightening between Gao's government and the UNSC, Special Inspector Veta Lopis of the Gao Ministry of Protection was assigned with investigating the serial killer on July 2, 2553. Meanwhile, the Insurrectionist-minded Gao politician Arlo Casille allied with the ex-Covenant faction known as the Keepers of the One Freedom to prevent the UNSC from gaining control of the Forerunner technology in the caves. Seeking to drive out the UNSC forces without forcing Gao itself into a war, Casille smuggled the Keepers on the planet, where they attacked the UNSC research battalion, resulting in several days of fighting in the Montero region. In the end, the UNSC forces evacuated from the planet with the Forerunner AI, Intrepid Eye, while destroying the Montero Cave System with a HAVOK tactical nuclear weapon. Casille chose to cover up the true source of the nuclear explosion rather than face pressure to enter into a war with the UNSC he doesn't want.
Over the next several months, the vengeful Keeper dokab Castor engaged in extensive piracy against Gao. The desperate Casille made a deal with Dark Moon Enterprises where in exchange for allowing Dark Moon the use of one of Gao's labs, Dark Moon would provoke the UNSC into dealing with the Keeper problem. After the family of murdered admiral Graselyn Tuwa were brought to the lab for experimentation, Casille decided to sanction the experiments rather than admit to the UNSC what was going on and face a possible invasion. After killing the Tuwas and cutting out their organs for Intrepid Eye's bioweapon plot, Dark Moon deposited the bodies on the Keepers' Salvation Base to frame the murders upon them. As planned, the UNSC hit the Keepers hard in Operation: RETRIBUTION, destroying Salvation Base and ninety percent of the forces stationed there. However, Veta Lopis and her Ferret team recovered the bodies and realized that the scene had been staged. Forensic evidence led the Ferrets and Blue Team back to the lab on Gao which had by that point been destroyed by Castor who had learned the location from captured Dark Moon operatives. The Ferrets and Blue Team confronted Casille who realized that Gao had been set up to take the fall the same way the Keepers had. Casille pointed the Ferrets and Blue Team towards Pinnacle Station above Meridian as the station's AI Administrator Sloan had suggested Dark Moon to him and was apparently involved in the setup.
Gao is a lush, jungled planet; viewed from space, the planet is largely mottled green fading to gray at both poles, which are covered by large deserts. Gao's jungles and caverns support a rich ecology with numerous native species (including saurios, frilled salamanders, idjoms, jungaloons, ribbon-snakes, no-shell snails, speckled jungle dragons and glowfish), while the planet's polar barrens are inhabited by great gray apes.
Gao was an insurrectionist hotbed as early as 2526, with the Gao Liberation Force forming around then. Eventually, the colony would go "free" and form its own government; the Gao Republic, an entity that considered itself sovereign. A second insurrectionist faction named the Committee to Preserve Gao Independence formed in 2553 after the UNSC began occupying parts of the planet.
The planet may be named after a city of the same name in Mali.
List of appearances