Navigating Cooling Tunnel D
As you enter Cooling Tunnel D you will find a pair of Joker's thugs -- one regular and one armored -- who have one of Two-Face's thugs strung up above a boiling pool for the Heat Exchanger in this part of the Mill. You may find it interesting to listen to their nasty threats as they try to intimidate the prisoner into cooperating -- but it seems that their prisoner is more afraid of his boss Tow-Face than he is of being dropped into a pool of extremely hot water. You know it cannot be easy to inspire that level of loyalty in an underling!
Once you are satisfied that you have heard enough, take out the first two thugs -- what I ended up doing was a Ninja approach and quick takedown on the regular thug and then a fast "B" plus rapid-fire 'X' on the armored thug to deal with them before stepping into the side of the room to destroy the Harley Quinn Statue that is located in the corner.
At this point you have a decision to make -- you can rescue Two-Face's thug if you want to -- or you can leave him hanging there. I chose to rescue him because that is what The Batman would do, but you should bear in mind that even though Two-Face's thug was clearly grateful to you, after a very short delay while he thinks about it, he quickly decides you have to die and attacks you! Obviously you make short work out of him, and add the pittance of XP he is worth as your reward for caring enough to save him...
-- Obtaining a Particularly Difficult Trophy --
This room has a few surprises in it -- and we are going to deal with them because we are here and it is convenient... But this is complicated so as you can see I made a separate video all its own for this part... I am going to explain this in detail, but you will want to watch the video so that this all makes sense to you as well, umkay?
First stand by the pool that the thug was suspended above and turn on Detective Vision to spot the round hatch on the blast furnace placed directly across from the pool. To the right (facing the furnace) is a barred area in which there is a fuse box and the aforementioned Trophy that this little mini-section is all about! Select the Batclaw and Grapnel the hatch, pulling it open.
How to Unlock this Particularly Challenging Trophy
Now select the Remote Batarang and turn to face the left wall at the top of which is a walkway that leads to a passage that is blocked by a closed and locked gate. The thing is at the very top of this gate and chain-link wall where it should meet the ceiling -- it doesn't -- and while that space above is inadequate for you to pass through yourself, it will admit a Remote Batarang. That is a good thing as it turns out because on the other side of the gate after you make a left turn in the passage there, you will find a pair of conductors with live electricity flowing between them.
What you want to do is launch the Batarang, send it through the narrow gap at the top, and then into the flow of electricity -- pull the Left-and-Right Triggers of your controler to cause the Batarang to reverse directions, and fly it back through the gap into the room you are in, and then fly it into the blast furnace, make a hard right-hand turn, exit the b;last furnace in the barred off area, and strike the fuse box with the electrified Batarang, thus short-circuiting it, and causing the bars to open! Walk inside and take the Trophy which, I freely admit, was not the easiest one to acquire, and then pat yourself on the back mates because hey! Well-bloody-done! Good-freaking-job! I am in awe of your skills with the Batarang -- your mad skills indeed -- and you should be very proud of yourself!
-- Moving On --
Now that you have obtained what has to be the most difficult Trophy in the game, blow the weakened wall with an Explosive Gel charge (if you have not already done so) and walk to the edge of the pool inside. This is another tricky part as we are about to take care of two more of Riddler's Secrets that are in a not-so-obvious place...
Throw a Freeze Grenade in the water and jump on the ice raft after it forms, then look up and to the left for spot that you can Grapnel to and RB up there. You are now in a chamber above this chamber, and on the north side by the chain-link blocked window/door there your next Trophy! Go pick that up but do rush off to leave just yet because remember this is a little complicated...
Walk over to the edge of the opening that you entered this chamber from and drop a Freeze Grenade onto the water. Now drop down onto the ice raft that it forms and turn Detective Vision on, and look back towards the room with the furnace as you will see that the top part of the Question Mark is visible in the distance, while the bottom part is on the edge of the ledge you are facing! As you slowly move down the stream these will line up, and you just need to scan it then to solve the riddle "This question can only be answered from an unusual perspective. Can you get yourself there?"
-- Navigating the Water --
At this point we are only going to pick up Secrets that are convenient to grab because it is now obvious we will have to return here later to grab the rest of the Secrets and solve the other Riddles. Sorry about that...
Using the Grapple-Points work your way past the blades on the right and duck under the obstruction, then use the Grapple-Points to enter the alcove on the right side of the stream. Jump onto the ledge and use the Line-Launcher to cross the long gap, then a Freeze Grenade to close off the two steam pipes that block the path ahead. This brings you out on the other side of the obstruction and once again facing the water.
Standing on the edge of this ledge here we are going to be doing another electrified Remote Batarang puzzle, only this time it is a lot easier! Ahead of you is a corkscrew-type gate -- fly the Batarang through that and into the electricity beyond, fly it down that hallway a bit before reversing direction, and then flying it back but through the corkscrew-type gate on the right instead of the one on the left you used to get in here.
Inside the area just past the right gate is the fuse box that you need to short-circuit with your Batarang, so do that and the corkscrew-type gate opens! Either use the Line-Launcher to cross the gap (you have to aim high and fire it as it lowers and you will then fall but catch the edge of the ledge on the other side and pull yourself up) or you can drop an ice raft and use that -- whatever you are more comfie with.
On the other side drop an ice raft into the water there and pull yourself along -- to the alcove on the left hand side which you should enter quickly as death awaits further downstream!
The Route to the Smelting Chamber
-- Out of the Water --
There are two ways you can go here -- the first is a ledge above you can RB to, the second is a duct vent cover you can use the Batclaw to rip off the wall above and to the right. Go tot he vent first by ripping it open and using an ice raft to get close enough to RB up to it. Inside the vent is a small room with a Trophy -- grab that -- and then turn on Detective Vision to spot the weakened wall here!
Blowing the weakened wall leads you back out to where you entered the area from nearer to the corkscrew-type gates where you can conveniently drop a new ice raft, so do that now and then RB up to the ledge further down stream that we did not do a minute ago!
Up here is a console that we can hack that will open the door and lower a bridge behind us. The bridge leads to the path to the Smelting Chamber, which is our destination, while the door leads back to Cooling Tunnel B. If we were seeking the rest of the Secrets we would probably head back to Cooling Tunnel B to cover the parts we were not able to reach, but as that is no longer the plan, instead we will head for the Smelting Chamber and our date with The Joker!
This area should look familiar as we have been here before -- as we approach the crates the doctor tries to kill us -- but realizes it is us and not a bad guy. She reports what she has seen, including the Joker placing mines ahead. You send her back into hiding and now it is time to enter the Smelting Chamber through the set of doors ahead of us!