By the power of truth, I, while living, have conquered the universe.
Posted by: goldsundragon
Sorry, but that is is no way legible English. Allow me to re-iterate that for you.
The Portal to the Ark is located under New Mombasa, this is the city of which ODST and a few levels of Halo 2 take place in.
The Prophet of Regret fled New Mombasa when he discovered the location of Installation 05 (aka Delta Halo), this is where ODST takes off from where Halo 2 leaves.
By the end of ODST, the Prophet of Truth's fleet emerges from Slipspace in full force and begins to glass the city in order to uncover the Portal.
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The Ark is where the Halo Rings are created, the planet in the middle (aka the Foundry) provides the minerals and resources in order to build the structure. The Halo Rings and the Ark were created by the Forerunners, an enigmatic race at war with the Flood 100,000 years ago. Long story short, they were losing, catalogued sentient species, retreated into their Shield Worlds, the Didact fired the Rings and the Flood went dormant.
The Forerunner then left their Shield Worlds, reseeded the lives they'd taken and imposed exile upon themselves for the atrocities they'd committed. So they left the Orion Arm of the galaxy in search for a new home.
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One such Shield World was called Onyx, a far-away world which was used to secretly train the Spartan IIIs. The planet itself had an outer core comprised of trillions of Sentinels which formed the crust, inside the Core Room Antechamber was a Portal leading to the Micro Dyson Sphere - this would lead to an area in slipspace where the Forerunners could be safe (as the Halo array didn't affect things in slipspace).
History lesson, done.