Upon viewing the new Halo 3 Teaser Trailer, and knowing what I know about the new achievements, immediately I came to a conclusion about what Bungie was about to release.

What I'm about to write is complete conjecture based on the data available to basically everyone right now.

The first things that stuck out for me about the trailer: This is New Mombasa. Last seen in Halo 2's campaign, New Mombasa is a massive Spaceport metropolis. What we see in the HUD is either some kind of surveillance camera, or an information visor of a robotic-type of entity (based on the movement). In the opening of this recording, there are several things of note:

1) Signs saying this is New Mombasa, 'orbital ladder' confirms.
2) Signs depicting a recent evacuation (HUD confirms).
3) Empty street visuals, but audio of firefights in the distance.
4) An incoming pod, hard to tell its origin. I would guess it's a UNSC spartan pod, or possibly a covenant pod.
5) Prominently, "TANAGA.0086", second clip is "LUMUMBA.0624"
6) Time is 4:11pm

Several things happen when the pods come down.
- Camera loses its Link, and displays "WARNING! METROPOLITAN DISASTER!" and "ALERTING EMERGENCY SERVICES!"
- City is wrecked.
- The last clip before a camera gets smashed, the designation of is "MBARAKI.2552", and 4:05pm. Which, strangely, is 6 minutes before the earlier clip of the same camera, before it was destroyed. ??

That was the Covenant attack on Earth in October, 2552. Master Chief climbs into his MJOLNIR6 on the same day, and attends a ceremony (which is at the end of Halo2).

Okay, so we've established what this is about, mostly. The question of who the pods belong to is still a little hazy. (I think it's UNSC).

So, Why? In the Halo 2 Collector's edition Bonus DVD Interviews, I came to understand that Bungie had a very hard time making Microsoft's launch schedule, and was forced to cut off at least 'several' levels from the end of Halo 2, which was supposed to be Master Chief Returning to Earth to fight the Covenant in one last, epic battle against the Prophet of Regret.

These levels never came saw the light of day, along with a lot of other stuff we eventually saw in Halo 3. With the new Title Update comes new achievements for finding skulls in levels called Heretic, Citadel, Assembly, Sandbox, Orbital and Longshore. Easily, I can see some of those referring to New Mombasa - especially Orbital, Longshore, and Citadel.

I think you can see where I've gone with this. Halo 3 Mythic could very well be the lost levels of Halo 2, extending the story back onto the time between Halo 2 and Halo 3 during the Covenant war on Earth, and after the wake of the slipspace destruction of New Mombasa. I'm guessing, because the Achievements are tied into Halo 3, that it's either a monster DLC pack, or possibly a Disc Expansion.