Welcome to Old Clockworks C-2, Underground Expedition. If you've been following our guide, there will be 3 Gem Stones that we recommend grabbing here that you haven't already found, as well as one Boo to track down.
Like in the first mission, flash the light sensor to raise the gate, allowing you through. Now head around the machinery in the center to a mound you can interact with. Luigi will put the Special Compass in place revealing an entrance to the Storm Cellar below. You'll find a couple of ghosts down here, including one underneath the bed. As for the one carrying the rock, you can simply use your vacuum to pull it away. With the room clear, use your vacuum to rip off the wall-colored sheet behind the bed to reveal a painting. Then use your Dark Light on that to reveal the mansion's 4th gem.
Now in order to open the gate blocking the door, you'll have to weigh down the lift it's attached to. Simply grab the rock that the ghost was carrying and place it on the lift to open the way leading into the Canyon Hall. Inside, head right and rip off another wall-colored sheet at the turn to reveal an exit leading to the Canyon Narrows area. Now grab hold of the dangling chain to use it as a zipline to the other side, then do it again with the northern one to cross back over into the Canyon hall. Now if you try to exit through the northern door, you'll find your path blocked until you clear a ghost from the area, who'll emerge from one of the nearby boxes. Once clear, there will be a gem stone nearby you can collect. To get it, look for an easily missed passageway on the right, just before the northern door, then go through the door there to reach the Canyon Stairs. Inside, look for the mansion's 5th gem trapped in a web high up the right wall that you can pull down with your vacuum.
Now return to the previous room and continue through the northern door into the Roundhouse Pit. Grab the chain attached to the wall ahead and give it a good tug 3 times to knock down the wall. Continue through the door that's revealed, which leads to the Pit Slide--wheee. When you slide down this puppy, you can use your vacuum to speed up or the blow functionality instead to slow down. Either way, continue through the door at the bottom into the Quarry. Follow the path down to a mine cart that you can pull using your vacuum. For now, tug on the attached chain just twice to pull it out far enough so you can enter the tunnel it came from. You'll find the mansion's 6th gem inside, which you can pull down with your vacuum.\
Return to the cart and tug it twice more to have it plug the hole that the sand's pouring into, which will cause it to start filling the area instead, raising the floor. Now as you ride it upward, you'll have to take down several swarms of ghosts, but this shouldn't pose much of a problem. Once you reach the top and clear the area, you'll be able to find this Mission's Boo. Follow the path to the gap, and use your Dark Light there to reveal a bridge, exposing the Boo.
Afterward, exit through the door on the right into the Antechamber. Here, you'll find a series of large webs blocking the hall. To clear them out of the way, grab the silk sac just left of the first one and set it aflame via the torches by the room's entrance. Now just carry it down the hall, burning everything that gets in the way. Who knew Luigi was such a pyro? Eventually, you'll reach the Tomb. Interact with the object in the middle to find one of the missing clock hands, but it's not yours just yet, as a ghost will run off with it. Go figure, right? In the meantime, a Mummy will appear that you'll have to take down. To deal with this guy, you'll first need to flash him with your strobe light to cause him to run in your direction. But the trick is that you need to make sure he's facing a wall, and is close enough, so that he runs right into it, knocking himself out. Then while he's down, you'll have to use your vacuum to rip off his bandages, exposing the mummy for what it actually is...a ghost Take him down like how you normally do. But once you do, two more mummies will appear--defeat them in the same way. Once you do, the mission will be complete.
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