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The Hollow Tree is a room inside of the Haunted Towers in Luigi's Mansion: Dark Moon. The Hollow Tree can be accessed by going north in the Courtyard and spinning the Pinwheel Gate so it opens. The Hollow Tree is a large, four floor room. The bottom of the room is covered with water and three alcoves on the left, right, and middle have spider webs blocking them. The bottom door to the north leads to the Tree Root. At the top of the room is a large door that can only be opened with a special key that leads to the Eerie Staircase.
During Graveyard Shift, Luigi must make his way to the Hollow Tree in order to reach the top where the boss lurks. Choosing the wrong path on the stairs causes Luigi to be automatically put in a fight with a Slammer or a Greenie. At the top, the door closes and Luigi must use an alternate route to get to the boss. In Tree Topping, Luigi will be pixelated to the Hollow Tree for a brief moment so he can open the door to the boss. In Hostile Intrusion, the stairs at the top are still pulled in, so Luigi cannot return to the Eerie Staircase.
Starting from Graveyard Shift and most missions after, Luigi must use the spider web balls and light them on fire with the torches here. After doing so, he can take the burning wad of web to the left alcove, which will burn away the webbing and reveal a bucket. After filling this bucket with the water in the pond, Luigi can water the plant on the right alcove and shine it with the Strobulb to reveal an emerald.