From the Super Mario Wiki, the Mario encyclopedia
To enter this room, Luigi will need a key, which he can find in the Waterwheel Room. There is a cell wall splitting the room in two. The doorway to the Bridge Room is locked and the key needed to unlock the door is in the cell. On the west side of the room, next to the door, is a rope that Luigi can pull with the Poltergust G-00. Pulling it causes the cell door to open. Gooigi can pull the rope while Luigi enters the cell and picks up the key. When Luigi does pick up the key, a Goob will come and splash water onto Gooigi causing Gooigi to melt which in turn locks Luigi inside the cell. The walls on either side of the cell, which have spikes on them, then start to close in on Luigi. To escape the cell, Luigi must send Gooigi out of the cell. Gooigi must then defeat the two Goobs outside of the cell and then pull the rope which opens the door, freeing Luigi. This only happens the first time Luigi enters the room.
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