The Ark (level)
From Halopedia, the Halo wiki
The Ark is charged with meaning in both Halo and the real world.
The "Ark" was a safe place holding every living being during the Great Flood that God created to destroy evil and create a new and sinless world.
This sounds familiar to some degree.
Could it be that the only place that the Forerunners were able to keep the Flood from infesting was the Ark?
The Ark, as stated in the games, is a place where all the "Fortress Worlds" could be remotely activated, destroying all life able to be consumed by the Flood within 3 radii of the galactic center.
It is commonly stated to be found on Earth, the homeplanet of the humans.
In the Halo 3 Announcement trailer, there is a massive Forerunner artifact found buried in the Earth's crust in Africa, near New Mombasa.
Could it be the Ark?
Halo 3 will answer this question. We'll just have to wait and see.