Portal at Voi
From Halopedia, the Halo wiki
The Portal is a Forerunner complex on Earth, buried under New Mombasa. It was uncovered by the Covenant during the Second Battle of Earth.
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 jumped into Slipspace within Earth's atmosphere. The Covenant fully excavated it shortly after they conquered Earth. According to messages from a Forerunner known as Librarian contained throughout the Ark, the portal on Earth was the last of numerous gateways constructed throughout the galaxy, and was buried shortly before the activation of the Halo rings.
The High Prophet of Truth landed the Forerunner Dreadnaught on the forerunner structure, and an unusual storm started up above it. The UNSC military unit ONI Recon 111 reported to Commander Miranda Keyes that the "cruisers above the crater have stopped...they've found something"[1]. The UNSC attempted to attack the structure, which was assumed by the UNSC to be the Ark. A small strike force in the town of Voi, consisting of SPARTAN-117 and some Marines and Factory Workers to clear a path for three Frigates and numerous Longswords.
Truth then activated the Gateway, opening a large Slipspace rift above the artifact which leaded to the Ark ,which is revealed to be 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 pursued them to the Ark.
The Portal is estimated to have a diameter of 25,515 meters [2].
