Halo 3 is set in a science fiction setting during the years 2552 and 2553. Humanity is at war with an genocidal alliance of alien races known as the Covenant. After years of conflict, a Covenant fleet discovers Earth during Halo 2. "Halos" are massive ringworlds, ranging from thousands to tens of thousands of kilometers in diameter, scattered across the galaxy. These rings were constructed thousands of years ago by a race known as the Forerunners as weapons of last resort against the parasitic alien species known as the Flood. When activated, the Halos would destroy all sentient life in the galaxy, depriving the Flood of its food. The Forerunners disappeared after they activated the rings. In Halo: Combat Evolved, whilst fleeing the Covenant, the UNSC ship Pillar of Autumn stumbled upon one of these ringworlds, Installation 04. Against the wishes of the ring's artificial intelligence (AI) caretaker, 343 Guilty Spark, the human supersoldier Master Chief destroyed the ring to stop the threat from Halo and the Flood. The Covenant, unaware of the destructive nature of the rings, attempt to fire another ring, Installation 05, during Halo 2 in order to fulfill their religious prophecy. One race in the Covenant, the Elites, learn the truth about the rings, and join forces with humanity in order to stop the installation's firing. Though they are successful, the unexpected shutdown triggers a fail-safe protocol, priming all the rings for firing from one location, referred to as the Ark. Still oblivious to the true nature of the rings, the Covenant High Prophet of Truth and the remaining loyalist Covenant proceed to head to Earth, where they believe the Ark is buried.