Portal at Voi

Were you looking for Porta, a Flood door? The Portal is a Forerunner complex in Kenya, Africa, buried outside the city of Voi. 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, although the artifact in the trailer is located outside New Mombasa rather than Voi. The Covenant excavated it shortly after they subdued West Africa. 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 electric 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". The UNSC attempted to attack the structure, falsely presumed to be the Ark by UNSC intelligence; but was unsuccessful due to heavy Covenant loyalist anti-aircraft fire batteries. A small strike force in the city of Voi, consisting of Master chief and some Marines and factory workers neutralised an AA battery to clear a path for three Frigates and numerous Longswords. These then made several attacks on the Dreadnaught with MAC Cannons and Air-Air Missiles, but with no effect.

Truth then activated the Gateway, opening a large Slipspace rift above the artifact which led to the Ark ,which is revealed to be outside the Milky Way galaxy. Truth and the rest of the Loyalist fleet advance through the rift as the Separatist fleet, along with the Arbiter, Miranda Keyes, and SPARTAN-117 pursue them to the Ark.

The Portal is estimated to have a diameter of 25,515 meters. Its centre is located between 3°15' and 3°20' South and between 38°30' and 38°35' East

Trivia
There are 14 monoliths that lift up after activation of the portal is another reference to the number 7. 14/2=7