Portal at Voi

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The Portal is a Forerunner artifact in Kenya, Africa on planet Earth. It is buried at the town of Voi. It was uncovered by the Covenant during the Second Battle of Earth in late 2552 or early 2553.

Summary
The storm was first seen in the Halo 3 announcement trailer when the camera shows the artifact being uncovered & activated. The storm lies directly over it, lightning bolts are seen flashing out of the clouds and the formation seems to have an eye, like a hurricane would. When the portal is activated, the beam travels up through this eye and disappears.

In the level Crow's Nest, when a player looks up at some of the computer monitors, you will see a map of the local area. The map will have a mass of clouds beginning to show a swirling motion around the area. When you enter the level Tsavo Highway, you can see a donut-shaped mass of clouds above the artifact that the Covenant are excavating. When you proceed to the level The Storm, the storm has intensified and grown. The clouds now bring rain to the area, and certain parts of the cloud glow as if cloud-to-cloud lightning is occurring. When Truth activates the Portal, the solid blue beam shoots up into the cloud formation's eye and opens the portal to the Ark.

After the portal is opened, and the level Floodgate begins, the remains of the clouds are seen broken up and swirling around the portal in the sky, having been disrupted by the opening of the portal itself.

History
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 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". 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 to Air Missiles, but with no effect. Truth then activated the Gateway, opening a large Slipspace rift above the artifact which led to the real 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 Thel 'Vadam, Miranda Keyes, and SPARTAN-117 pursue them to the Ark.

The Portal is estimated to have a diameter of 25.515 kilometers using an overhead image and the size of the ship that circles the Ark Portal itself,.

The Storm above the portal is an artificial storm, created only by the activation of the Ark Portal.

Trivia

 * On Behind the Scenes on the Halo 2 Collectors Edition, there is actually a sketch of The Portal upon activation.
 * At a point in the level The Storm, a group of UNSC Marines are present around a radio. Intel comes over the communications channel and says,"...winds inside the storm have been clocked at over 200 kilometers per hour."
 * The Portal was 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 had 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 is 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 (7+7=14). The entire structure seems to be constructed of 7 identical parts, with 7|seven pairs of monoliths.
 * 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.
 * The location of the Ark Portal is marked on Google Earth, this location is misplaced, however, but a new location has been marked which is in the right place.
 * The monoliths bear a striking resemblence to the monoliths around the processing station from the film Aliens.