The Ark (level)
From Halopedia, the Halo wiki
The Ark is charged with meaning in both Halo and the real world.
In the real world, the "Ark" was a ship talked about in the Christian bible. The Ark was a safe place holding a breeding pair of every living being during the Great Flood that God created to destroy evil and create a new and sinless world.
In the Halo Universe it is thought that the Forerunners built an Ark to as a last resort against the Flood.
As stated in the games it 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.