Crow's Nest

Crow's Nest is the second campaign mission of Halo 3.

Upon completing it on the Normal, Heroic, or Legendary difficulties, the player will earn the "Holdout" achievement and 20 Gamerpoints.

Background
It takes place in a UNSC Base, which "comes under heavy fire" from the Covenant.

It is probably the location seen seen at 00:45 to 00:47 and 00:54 in the Halo 3 E3 2007 Trailer, showing a few Mongooses travelling through it and the Master Chief and Arbiter defending it with AIE-486H Heavy Machine Guns. At approximately 1:02 in the trailer, Drones can be seen picking up marines somewhere in the base, as well.