The Artifact

The Portal, also known as the Gate, is a Forerunner complex on Earth, buried under what used to be New Mombasa until the Slipspace Catyclysm.

It is the Forerunner artifact revealed in the Halo 3 E3 2006 Trailer. While it had been under New Mombasa, it was partially uncovered when the Prophet of Regret jumps into Slipspace within Earth's atmosphere. The Covenant fully excavated it shortly after they conquered Earth.

The High Prophet of Truth landed the Forerunner Dreadnaught on the Portal structure, and an unusual storm started up above it. The UNSC military unit ONI Recon 111, that was assigned to shadow a battlecruiser heading toward the Portal's crater, reports to Commander Miranda Keyes that the cruiser has "stopped...it's found something". The UNSC attempts to attack the structure, assumed by the UNSC to be the Ark, with a small strike force in the town of Voi, consisting of SPARTAN-117 and some Marines. In order to get UNSC ships to the complex, they destroy Covenant anti-air batteries and Wraiths.

Truth then activates the Portal, opening a large Slipspace rift above the Cartographer which leads to the true Ark structure, which is revealed to be extragalactic and outside the Milky Way galaxy. Truth and the rest of the Loyalist fleet retreat through the rift as the Seperatist fleet, along with the Arbiter, Miranda Keyes, and SPARTAN-117 pursues them to the Ark.