The Ark was a planet-sized Forerunner construct, much like the Halos. It was shaped like an eight-pointed star with a hollow circle at the center, and one of the flat sides was terraformed in the same way as the inner surface of the Halos. It served two siginificant purposes: Firstly, it served in multiple ways as a "headquarters" for the Halo Array. It was designed to automatically construct replacement Halos, open a slipspace portal to move them into position, and then remotely activate the Array. Secondly, and far more significantly, because it was positioned outside the galaxy and thus the operating range of the Halo Array, it acted as a "Noah's Ark" (which is probably, in the real world, where it got its name) for the galaxy. Specimens of each species of sentient life were collected and stored on the Ark in a state of biostasis. Then, after the Halos had successfully wiped out the Flood, they were returned to their homeworlds and reanimated, unaware that anything had occurred.