Go to Survivor Mode. In either of the areas with the boost-pad, you should see a small area with a morph-ball boost that you can't enter. To the left of this unreachable area, there is a small pillar. If you sneak in between the two, you should be able to just get underneath the pillar. Entering morph ball, moving backwards, or jumping will make you fall through the area. If you want to get stuck half in, half outside of the map, simply walk backwards underneath this pillar, and quickly move forwards. If done correctly, you'll be stuck. You can still fall through the map if you're not careful (by walking off the edge), and there are metroids as well as the other Xenomorph things that are, for the most part, killable. You're not really outside of the map, but rather seeing everything as blue, like you're inside the wall. To exit, simply return to the hallways with the zoomers.
Submitted by: Crad