The Ark (level)
From Halopedia, the Halo wiki
The Ark is the forerunners last safe haven. It is where all the halos (instalations 1-5) can be activated. It is presumably hidden deep somewhere in earth, possibly in the Bermuda Triangle. The Covenant are looking for it so they may commence the Great Journey. In Halo 3 your assumed mission is to prevent this, as it would lead to the annhilation of all sentient life-forms. You find out about the ark from 343 Guilty Spark at the end of Halo 2. He shows the arbiter all of the halos on a holographic screen, along with an earth-looking planet. The Arbiter points to it and asks what it is. 343 replies, "Why, the Ark, of course", where he proceeds to give you info about the Ark
