Jump into the Tunnel Glider and head downward. There's only one direction you can go, so enjoy the ride for a little while. Eventually you'll come to a locked gate. Exit the Tunnel Glider and use your trusty Laserkraftwerk to cut the chain. Return to your vessel and continue your journey.
The next gate you come to will have a small tunnel underneath it. Swim through to the other side so you can cut the chain. Get back in the Tunnel Glider and head through the narrow passage. When you reach an opening with a propeller on your left and ladder on your right, climb the ladder and press the button until the opening is positioned under water.
Tip: You can easily get lost in this next part. Pay very close attention to the directions we give you going forward. The pathway to your current objective is to your right, between the two sets of blades you had to adjust a moment ago.
Get back in the Tunnel Glider and go through the opening in the first set of blades. Your objective is just to the right, so keep this location in mind. Go through an opening in another set of blades, then turn left and go through another tunnel. When you reach a large opening, look for another tunnel on your right blocked off by some debris. Head toward the debris and exit your vessel to find a Gold Snake just at the base of the blockade. Return to your Tunnel Glider, then backtrack to the tunnel we showed you that contains your current objective.
Exit your Tunnel Glider and climb up to the ground. Go forward, climbing a ladder on your left, then one more. Head into the next room and hold the lever until the area fills with water, then head back to your Tunnel Glider one last time.
Navigate your vessel forward a very short distance until you reach a platform. Exit and climb up, leaving your Tunnel Glider for the last time.
Jump into the water, then swim toward the underwater mine. Use your Laserkraftwerk to cut the chain and clear the blockade. Think about getting some air, then swim through the opening and all the way to the other side where it gets deep.
Dive down and swim through the small opening, following the tunnel until you have to surface. Search the area for ammo, health and armor, but you can only go one way, eventually taking an elevator ride up to another room full of loot.
Tip: This room contains the Tesla Laserkraftwerk Upgrade. Make sure to snag it before moving on. It will come in handy later.
Use your Laserkraftwerk to cut your way out of this area, turning to your right and hugging the left wall of the tunnel. A short distance ahead, you'll get two Signal Detected notifications on your screen.
This entire area can be completed with stealth, and should be in order to avoid more trouble than you need bother with.
As you approach the platform, you'll have the option to climb on top of it or sneak underneath. Head up to the top, waiting for a guard to approach you. When he turns his back to you, take him out with your Knife. Continue to hug the left wall while using the cover to sneak up on the closest Commander.
Tip: The Commander has a Kampfhund near him. Take out this ugly beast first to avoid fighting him face to face.
With the Kampfhund down, sneak behind the Commander and execute a stealth kill, then start moving toward the next one. Be sure to stay in cover, waiting for the Commander to turn and walk away from you. Sneak up behind him and take him down, then do the same to the one remaining guard.
With the area clear, head back to where the Kampfhund and first Commander were located. Take note of the doors near the wall, exploring that area for some goodies. When ready, press the button that is just behind where the Commander was standing, causing a giant robot to come out and attack you.
If you have a fully charged Laserkraftwerk, you can take this guy out with no trouble at all. Once you put him down, press the button again, raising the gate all the way up and then sneaking under it before it closes.
You don't have any opposition here, so be sure to fully explore the area, picking up all of the goodies on the ground. When you're satisfied that you have everything, head to the main control room and hit the button on the desk. Of course, this will prompt an attack from your Nazi enemies. Stay in the room and thin out your foes from this location. You can pick off advancing infantry as well as toss Hand Grenades down on top of the heavily armored ones.
Make your way to the far end of the tunnel and hang out in a door that overlooks the main tracks. From here, you should be able to pick off any bad guys and use your Laserkraftwerk to demolish the larger robots. You will most likely need to charge your battery, so head down to the charge stations along the tracks and fill up. Once the last robot is down, head to the main train car to complete the final objective and the mission.