The Ark is a mysterious Forerunner facility from which all the Halos can be controlled. In the end of Halo 2, Delta Halo is armed and then unexpectedly shut down, causing it to send a superluminal signal to all other Halos in the galaxy, putting them on standby mode. 343 Guilty Spark informs Miranda Keyes and Sergeant Johnson that the Halos must now be detonated or shut down from "The Ark". It is believed to be either the Forerunner ship Master Chief boards in the end of the game, or somewhere on Earth.
It is rumored that Halo 3 will reveal more details about the Ark. It is also rumored that at the end of the credits of Halo 2, there is a reference to "February 9th". This may indicate that a downloadable level pack via xbox live will be made available to the public.