The Crow's Nest Entrance is a pit located in Halo 3's campaign level Sierra 117. The pit is placed beyond the level boundaries, and is the (incomplete) entrance to Crow's Nest, shown in Sierra 117's last cutscene.
In Sierra 117's last cutscene, Hocus's Pelican is seen landing in the pit, but the camera angle prevents players from seeing inside. The pit's interior is somewhat incomplete, and no actual entrance to Crow's Nest is there; this is because there was no need for Bungie to fully model the entrance, and so they didn't.
Start the level on Co-op, and get at least two players to the location of the Iron skull. You don't need to actually obtain the skull, and you can start at Rally Point Bravo to reach the area faster. Clear out all enemies in the area, but don't rescue Sergeant Johnson or the other Marines.
Have one player (Person A) walk into and face the corner opposite the skull, and have the other player (Person B) jump on top of them. Person B should face into the same corner. Person A should then repeatedly jump into the corner; eventually, some flaw in the level geometry should cause both players to "moon jump" on top of the wall.
At this point, one player (for our purposes, Person A) should stand on the grassy edge of the wall, where it meets the rock. The other player (Person B) should jump off the wall and into the water and the kill barrier that lies beneath its surface.
Person B should then respawn above Person A, before promptly being glitched past an Invisible wall and onto the rocky area. Person A should then jump down to their death, so that they can join Person B outside of the level's boundaries. Once the two have regrouped, they should walk up the cliff face, and upon reaching its "corner," jump onto the tallest building. They should then follow the top of that building, walking towards the dam; upon reaching the building's edge, they should jump onto an adjacent rock face and continue up it.
At this point, one person may stay behind if either fears death. We shall assume that Person A continues forward and Person B remains behind. Person A should follow the cliff face until they reach a caveman, at which point they should keep walking upwards.
Eventually, Person A should reach some grassy ground; by continuing up the hills, they will reach the entrance. Person B can kill themselves to respawn with Person A if they like, but it should be noted that anyone who falls in the pit will not be able to get out, so at least one player should remain outside of it.