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Conan O'Brien & Andy Richter Marines
On mission start head straight down and stop before entering the room with the terminal computer and turn left facing the Pelican on the rails. Jump over the rails and head left to the two marines standing near the crates looking at the Pelican from the far left corner. Now stand there and listen to the dialog, the Marine one is Television's Conan O'Brien, and Marine two is Andy Richter.
If you have any trouble locating the two marines in the large room then you need to watch the video provided below. Enjoy the Easter Egg.