Portal at Voi

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The Portal, after activation by the Prophet of Truth.

The Portal is a Forerunner complex in Kenya, Africa on planet Earth. It is buried outside the city of Voi, underneath the Voi river. It was uncovered by the Covenant during the Second Battle of Earth in late 2552 or early 2553.

It is the Forerunner artifact revealed in the Halo 3 E3 2006 Trailer. It appears as Spartan-117 looks down, and notices the ground opening up. 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 Portal, viewed during the day, prior to its activation by the Prophet of Truth.

The High Prophet of Truth landed the Forerunner Dreadnought 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"^[1]. 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 John-117 and some Marines and factory workers neutralized an AA battery to clear a path for three Frigates and numerous Longswords. These then made several attacks on the Dreadnought 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 ^[2]. Its center is located between 3°15' and 3°20' South and between 38°30' and 38°35' East.

Trivia

The portal, when activated cause a portal storm which can cause wind gusts of 300 kilometers near the center.
The Portal is originally both underground and submerged, lying under a massive human-made lake. By the events of Halo 3, the entire lake and the earth below has been vaporized by Covenant cruisers to unveil the Portal.
At the end of Halo 3 where Lord Hood is making his concluding Tribute speech about the Human-Covenant War and the Marines who died in the strife and war, you can see the Portal closed when the Phantom comes to transfer the Arbiter to the Shadow of Intent.
It is unknown how the Ark Portal was reactivated without the use of the Dreadnought. There may be an alternate method to opening it from the Ark.
The 14 monoliths that lift up after activation of the portal is another reference to the number 7. The entire structure seems to be constructed of 7 identical parts, with seven pairs of monoliths.
In the E3 2006 Halo 3 trailer, there is no Forerunner Dreadnought activating the portal to the Ark. But if studied carefully, you will note that in the trailer the Portal is half-way through activation. Players will realize that during the cut-scene in which the reveals Portal is activating, the Dreadnought "Keyship" actually lowers itself into a stoop into the Portal's center. This explains why there is no Dreadnought in the E3 2006 Halo 3 Trailer as the Keyship lowers itself into the Portal to "unlock" the contraption and gateway.
It has been further stated by Bungie that at one point, during story conception, the area that ended up being the Portal had originally been intended to be The Ark itself.