During the third part of the "Interlooper" level, get to where you see a Garg (a huge monster who can burn you) crushing rocks and stones. To be able to pass through, there will be a small hole behind. When you enter it, you will find a little waterfall and plenty of water on the floor. When you step into it, your health will gradually recover. Before entering the hole, look in the direction where the Garg crushes the huge rocks. You should be able to see a sort of smooth staircase-like slope. It will be on the left, stuck to the wall. Climb on it. When on top, look to the right to see a tiny lighted room with med-kits and ammunition packs. Duck, and crawl into it.