Start level 6-2 (Into The Abyss). Kill all the guards in this little area, but make sure not to lose any health. Go through the door directly ahead, then go to the left. Do not kill the civilian. Then go to the right to where the elevator blows and go into the intermission sequence. Run to the platform. Go down the ladder and shoot open the door so that the civilians can get out. Do not get off the elevator. Instead, go to the back right corner and stay there. You will fall with the elevator and lose much of your health. However, you will not be dead and can see through the walls. Note: The only way to exit is to die.