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The Shaft is a room inside of the Secret Mine in Luigi's Mansion: Dark Moon. The Shaft is accessed by taking the north door inside of the Smuggler's Hideout. The Shaft, as the name implies, is just a long shaft that spans between Floor B2 and Floor B3. Part of the ice walls surrounding the Shaft are cracked, allowing Luigi to see various things going on in the background. Near the bottom of the Shaft, there are some compartments that hold coins and bats. The door on Floor B2 leads back the Smuggler's Hideout and the door on Floor B3 leads into Coward's Chasm.
During D-2: Hit Rock Bottom, Luigi needs to use the Shaft to get to Floor B3 where he must investigate the commotion inside of the Crystal Quarry. While the elevator is moving down, Luigi can see a Greenie and two Slammers carrying large, red crystals. These ghosts cannot be stunned with the Strobulb and they ignore Luigi. However, there is a Gold Greenie right after the Slammers that can be stunned and captured. The Gold Greenie and the ghosts carrying crystals are not present in any other mission.
Starting on B-2: Hit Rock Bottom and most missions after, before the Shaft reaches Floor B3, there is a long chain hanging down from one of the compartments. If Luigi pulls this chain at the bottom and moves the elevator back up, the compartment is open, revealing a sapphire.