Get under the level on Fuel Depot
Walk into the large warehouse in the middle of the level. Walk in front of it see a large shelf with a few racks on the left side, in the middle of the warehouse. Shoot all of the loose objects off the shelf until you see a small yellow object. Look closely behind the shelf to notice a small indentation that leads to the left that you can climb on top of, leading you to a higher platform. Kick the yellow object into the top left corner of that indentation, then take cover against the object. It will appear as if you are taking cover against a wall. Climb over the wall and you will be under the level. Note: Once under the level you cannot get out.