This is The Ark, as seen in the cutscene that concludes Halo 3's last level. The large circular light, as well as the ring of light that surrounds it, is the incomplete Halo 04a firing. The much larger structure in the background with eight arms is the Ark. The Ark apparently serves many functions, not the least of which is a construction facility for Halo installations, as well as a master control room (seen in the level The Covenant, where the final confrontation with the Prophet of Truth occurs).

In the sky above the Ark during the final run to the Forward Unto Dawn in the final level, an apparently strip-mined rocky body can be seen; presumed to be the source of the raw materials used to build Halo installations.