The pressure plate in the hallway is activated by hostile creatures. It opens some hatch covers blocking the input side of the pumps directly north of the kill zone. Water floods into the hallway, which freezes, kills the goblins, and activates a second pressure plate on the right. The second plate allows some magma into the room directly beneath the hall, which melts the ice. It also opens hatch covers blocking some pumps on the layer above. The pumps on the layer above remove the (now melted) water and the trap resets. Retracting bridges are used to allow the water and magma to return to its original position.