This glitch requires you perform the Item Jumping Glitch, so practice beforehand!
Load the Chapter "Sandtraps" (It's directly after "Highway 17") and get to the point where you fight your first Antlion Guard. On the ledge BEFORE the loading screen that leads to the boss fight, Item Jump untill your over the hills and out of the level. Fly around untill you can see a box, hovering out there. Try to land on it. Its purpouse is unknown, but its supposedly a developer code object. (perhaps a skybox?)
Found by ArmoredScout