Mission: The Maw
At the very end of the game, you will encounter foe hammer at the bridge. Afterwards, Cortana will tell you to go forward to find the new docking bay. On your way there she will mention "a large gap" that will require full speed to cross. At this gap you have to aim slightly to the left, and land on a small platform that hangs of the side. Once you land, and survive, park your warthog next to the wall, and use it to jump up to the ledge. Once on the ledge, you can drop down into the centre and find two small room. Inside one of them, you'll find this Easter Egg and unlock the T-Rex achievement.