You can literally fall to the bottom of the world in one of the Xen levels

On the map bm_c4a3d1, there are some damage (and collision) problems that lead to being able to fall through the world. Directly after the section with the scientists in test tubes, you can fall through the laser “kill” zone (which only reduces and 80 suit energy), allowing you to fall through the map. (Video proof: https://gfycat.com/personalhotbobolink )