In the stage where you take pictures of the blueprints of the OH-IX, go to the area where there is a test engine that you have to destroy by pumping up the pressure level. Once the engine blows up the wall, shoot the two or three Nazis. You will enter a corridor or hallway where you will see boxes and Nazis shooting at you. Go straight to the end of that hallway. You will see a green door at the end, but do not make any turns yet. By that door, there is a lamp directly above you. Turn left to that little hall where you will see a box and another green door. Next, face back again to the opposite direction, where you will be facing a wall where the lamp mentioned before is. Walk straight into that wall to enter to a secret passage where you can see the rest of the stage. Its a 3D design of the stage. You can see the place where the "lava" is located, and can even see the end of that stage with the medical supplies, bazooka, rifle, and more. Feel free to explore, but you cannot walk top far because the walls will stop you.