Crow's Nest

Crow's Nest is the third 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 traveling 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.

Not much is definite about this level, but we can assume that at the end of Sierra 117 the Chief reaches this base, inside a cave, inside a mountain. It is likely that the Covenant assaults the base with a multitude of ground troops, but somehow, it is destroyed, possibly by orbital bombardment, as Tsavo Highway begins in the ruins of the base's garage.

Trivia

 * It is seen in the Halo 3 ViDoc Et Tu Brute around the 5 minute mark.
 * A three part video of the full level can be seen on youtube.