When you find the driller and get inside it, drive it out until you can get out and walk around to the back. When facing the back, jump up onto the back of driller. This may require several attempts. You will then be able to walk around on top of the driller, but half of you will be inside it. Keep moving around on the top until you fall in. You will now be able to walk around inside the shell of the driller. To get back out, go toward the front and press X to drive it then press X again and you will appear back outside of the driller.