Installation 00

The Ark is an enigmatic Forerunner device from which all the Halo Installations across the galaxy can be activated. In the end of Halo 2, during the final cutscene, Delta Halo is armed by Tartarus, but is then unexpectedly shut down by Miranda Keyes, causing it to send a superluminal signal, hypothesized to be constituted by tachyons, super-relativistic particles, to all other Halos in the galaxy, putting them on standby mode, primed to fire. 343 Guilty Spark informs Miranda Keyes and Sergeant Johnson that the Halos must now be activated from the "Ark".

Speculation
The exact location and characteristics of the Ark are unknown, but there is rampant speculation, especially in the wake of the Halo 3 trailer.

Most evidence leads to the location of the Ark on Earth, buried under the African soil near New Mombasa.
 * In Halo 2 the Covenant ship carrying the Prophet of Regret jumped into Slipspace, for unknown reasons, just a few hundred meters above New Mombasa. This caused the destruction of New Mombasa, and unearthed a huge Forerunner artifact seen in the Halo 3 trailer.  The Halo 3 Announcement Trailer depicts a huge structure firing a beam of white energy straight into the sky, through the center of swirling clouds.
 * In the end of Halo 2, the conjecturally-named "Forerunner Ship", carrying the Prophet of Truth, jumped into Slipspace from near High Charity and Delta Halo, and for unknown reasons, voyaged to Earth, possibly towards New Mombasa, as the Forerunner Ship was headed towards Cairo Station, which is in geosynchronous orbit over New Mombasa.
 * Within the Halo Graphic Novel, in the short comic Second Sunrise Over New Mombasa, a transmission is intercepted between the Covenant assualt forces stating that once the humans were dealt with in the Second Battle of Earth, they would start uncovering the Ark.
 * In the real world, a concept sketch found in the DVD (seen right) names the artifact as the ark.

While the above evidence points to the structure in the Halo 3 Announcement Trailer being "The Ark", it has not officially been announced as such by Bungie.

Further rampant speculation can be found at Halo.Bungie.Org's Ark forum.

Trivia
The name of the Ark may refer to the biblical story of Noah's Ark (Genesis, chapter 7) that was built to save Noah's family and a number of animals from a massive flood, "the ark saves all sentient beings from the Flood."

However, it's also possible that "The Ark" is a reference to the Ark of the Covenant. In the old testament, the Ark of the Covenant, which contains the stone tablets of the ten commandments, is the holiest object of the Jewish faith. (This is the same Ark that stripped the skin from the Nazis in the Indiana Jones movie). The Ark of the Covenant is popularly believed resting somewhere in east Africa.

Links

 * Halo.Bungie.Org's Ark forum.