Secret to level...18 I think it is. The one where you're on a platform that leads into a fire pit and GLaDOS talks about being cooked alive or whatever.
You don't actually have to use the platform. You can go from portal to portal by standing on the border between portals on the very edge and go through just far enough to shoot another portal. Once you get to the fire pit, and can see it, you can see on the other side a tiny strip of white. If you can shoot a portal at that tiny strip of white, you just turn around and fall up through the floor and on the other side of the fire.
Fun game!
