I just re-watched the Halo 2 cutscenes on the extra Legendary disc from the Legendary version of Halo 3 w/ the helmet and all, also with the directors commentary on, which I always turn on for this disc because I find it hilarious. But yeah so at the end of the final mission where you play as The Arbiter you kill Tarturus, the master brute from the Covenant...then a cutscene starts where Miranda runs towards the index (key that turns the Halo rings on and kills everything in that galaxy radius) and pulls it from the activating core, just before the Halo ring fires it is cancelled from the pulling of the index...Sergeant Johnson, Guilty Spark (the monitor "guardian" of the first Halo), The Arbiter (the special 007 agent of the Covenant that has allied itself with the Humans), and Miranda stand around the deactivated Halo's control center where Guilty Spark explains that in an emergency shutdown of a Halo ring a signal is sent to all the other Halo rings putting them in standby mode where they can be remotely detonated from an Installation called The Ark. This was the big surprise moment of the ending in that there was a place called The Ark that no one had known about until the very end of Halo 2. Anyway, the scene changes to a shot of Earth's orbital defense stations under fire from Covenant battleships and at this point a Forerunner Dreadnaught ship (the main spaceship inside the Covenants Holy City that powers the entire city) leaves warp space and a broadcast from a human on board this Covenant ship is isolated by the human General on board one of the orbital space stations...The General realizes its Master Chief and asks "Would you mind telling me what your doing on board that Ship, Master Chief?" and Master Chief replies in a big close-up "Sir...Finishing This Fight." Music plays and the credits role.

This was majorly disappointing to alot of people because Halo 2 had been such a good game but it ended on such a bad note and cliffhanger. This mostly happened due to time constraints on the development teams side because during the developer commentary they are about to explain what was SUPPOSED to happen at the end of the game (there was supposed to be a whole level that got cut) and then realize that they shouldn't say what it was because it would've had an entirely different impact as to what happened in Halo 3 and would've changed how the story went. So yes the main thing though at the end of Halo 2 was the revealing of The Ark, and so during all the E3 world premier trailers of Halo 3 you see a large structure that is unburied on Earth and alot of people thought that this WAS The Ark..little did they know that this was just a structure from Earths ancient history that created a portal to where The Ark actually was (outside of the Milky Way Galaxy and completely safe if the Halo rings were fired and killed everything in the known galaxy).