Area 2-2: Museum Vault
The level starts at the vault. You'll have to go through the entire basement level of the art museum just to open it.
When you enter the door, go right, as there will be three big self-repairing barriers in your way, forcing you to head in the other direction. As you go, you'll see enemies as well as chips, and you'll also be introduced to pink ghost-like enemies that move forward and chomp at you like some ethereal Pac-Man. You'll need to get away from them before they run into you, but at least they don't automatically kill you on contact. It also helps that they can only turn in a different direction via the drill ducts.
The steam that comes out of the pipes will damage you, so watch out for it.
You'll soon enter a guard station, and if you move to certain things in the room, you can read them. One of them is a card on the wall that tells you to turn the red dial two times to the right and four times to the left. Once you latch onto the machine in the room with the drill, you'll be able to turn the vault.
R will turn the dial right, and L will turn the dial left. Hold down the button you need to press to turn the dial the right way and keep it in first gear to make sure you get one complete revolution in that direction. When you turn a dial in any correct direction the correct number of times, the light will flash a different color (in this case, yellow), reminding you to turn the dial in the other direction. Keep that in mind. If you mess up, you can press Start to restart. When you get it right, you'll get the confirmation jingle, the lock will be released, and the machine to turn the dial will disappear. You can then continue on.
When you reach the fountain, you'll meet Detective Carrie, who will throw EMP bombs in your direction. Head to the right and latch on to the machine to move the jet of water that moves up and down to knock the bombs out of the air and destroy them. The aim is to take out as many EMP bombs as you can before Detective Carrie eventually runs out. L moves the water left, while R moves it right. Make sure to shift up to second gear to give yourself the ability to move the water more in one direction in case you need to take out a ton of bombs. After the detective runs out of bombs, she and her forces will head to the vault. Grutch will tell you that the Red Diamond is in the vault, while Gearmo will tell you to use the jet of water to continue on. Do just that to head to the right.
You'll soon find another office as well as a note saying that the Skullkers are paying the force well to focus exclusively on the Red Dozers, which more or less reveals that the police force here is easily corruptible. You'll also find a poster for a display for five diamonds, and below it is the combination for the machine that is in the room. To release the second lock, you'll need to latch onto the machine and turn the dial five times to the left and three times to the right. While turning the second dial, you'll get a light blue light when you turn the dial five times to the left, reminding you to switch directions. Once you release the second lock, you'll be able to get third gear.
You'll drop back to the fountain. This time, use the fountain to head left. There will be a self-repairing barrier that will block your path when you head up and to the left, forcing you to go right at first, but we have third gear now, so we can break it.
To get past the six lasers, move to the left, then slide when you reach the middle of the laser set.
You'll soon need to use both the socket lifts and the drill ducts together to get further up. There is a cracked wall to the right that I (very easily) miss, which leads to a treasure that I could have picked up at this point in the game. We'll pick it up later. To be fair, it is easy to miss that cracked wall.
After making our way back down (and sliding through a narrow corridor along the way), we'll find a painting we have to move to get some blocks out of our way. There will also be a bookcase above where the blocks are, and if you interact with the right half of it with the B button, you'll find out that there is something odd about the diamond that makes whoever holds it covet it. You'll also get the combination to the third lock, which is a bit more complex: six times to the left, one time to the right and twelve times to the left.
The third dial is bigger than the first two, and to ensure that you get one complete revolution, shift up to second gear, then hold down the button until second gear is over. You'll get a green light (which can be a bit difficult to see, but it's there) when you turn the correct direction the correct number of times. Once you release the third lock, you'll open the vault. Go down the nearby drill duct, then head left once you're back outside.
After some more drill duct crawling, you'll fight a mechanical crab inside of a duct, putting your drill duct skills to the test. The eye that flashes indicates which claw it will attack with, and you'll need to attack that claw to damage the boss (or avoid that claw if you're not in position to). If it's the left eye, it will be the left claw; if it's the right eye, it will be the right claw. When the crab lowers its eyes into the duct after you successfully avoid its attack, you'll want to attack the eyes with the drill.
You'll soon end up back where you started exploring the level, and you can return to the vault. Latch onto the machine and keep drilling with the R button to open up the vault and go inside. When you do, you'll find out that there is gold bars in the vault, but no Red Diamond. You'll then meet Detective Carrie, who produces a yellow diamond instead, which she uncharacteristically covets, much to the surprise of Gearmo. You'll then have to fight Detective Carrie.
For the first phase of the battle, avoid the handcuffs that Carrie throws. She can throw them high or low, and they can also return to her like a boomerang on occasion. If you get caught, you'll have to drill your way out of the handcuffs. The middle of the robot will then open up and flash. You'll want to drill into it and shift up to third gear to do as much damage as possible.
For the second phase, wait for Detective Carrie to dive. The undercarriage will flash, and when she dives and then drops down, you'll want to jump up and drill upwards into the robotic torso. All the while, guards will pop out of the gold bars to make things more difficult. They don't pose much of a threat if you stay focused on Detective Carrie, though.
Once you beat the second phase, Detective Carrie will come to her senses. Strangely, you'll take the Yellow Diamond. Meanwhile, in the trailer, Gearmo will bring up the five diamonds, and mention that there is a blue diamond in the Kuru Ruins. With this as our only clue to go on, we head to the Kuru Ruins to look for the Blue Diamond.