You head right when going through the now-open-door and you see a gigantic shaft leading straight into the center of Halo. This will be important later on.

When you head left, you get into a series of enemies. In the first area, go up the ramp so you can shoot them all from above. Go down and gather up ammo - running out of ammo is a problem in this section as well. Now go through the doorway across from the ramp area.

You'll go down a spiral walkway, and through another door. This area has a bunch of enemies including 2 big enemies guarding the final area. Blast through them all, being sure to dodge. Again, the bigger enemies will stay back until they are near death, at which point they'll charge you. You can get them in 1-2 hits each if you have the rocket launcher from the beach.

You now hear the marines are under attack. Keep going. You'll go down a spiral and then be out on the eldges. A door to the left is a dead end but gets you active camo. A door to th eright has enemies. You go along a ledge and find the Cartographer. Now you race back out the way you came, and get picked up.

They fly you down into the shaft. Master Halo Walkthrough