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 is inside a cave, which "comes under heavy fire" from the Covenant Loyalists.

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,then you must pick a warthog and fight the covenant loyaltist until you reach Tsavo Highway

Trivia

 * It is seen in the Halo 3 ViDoc Et Tu Brute around the 5 minute mark.