Under the Ice
From the Super Mario Wiki
Under the Ice is a room inside of the Secret Mine in Luigi's Mansion: Dark Moon. Under the Ice is accessed by dropping down from the Fishing Hut or by taking the door on the right inside of the Terminal. The room is completely covered in snow and sheets of ice. There is a treasure chest here and geysers of cold air that take Luigi to a higher area of the room. A large fish head is found here, presumably from the Fishing Hut above and there is an abandoned mining shaft nearby. The door to the left leads into the Terminal.
During Cold Case, Luigi falls down through the floor of the Fishing Hut after the Toad assistant breaks it. Professor E. Gadd calls Luigi to see if he is all right, but reminds him to use the elevator in the Airway to get back to the Chalet. In this mission, if Luigi pulls out the two fishing lines that are hanging from the large fish's mouth, a Gold Greenie appears.
Before Luigi and the Toad assistant go through the door to the Terminal, there is a shaft nearby. If Luigi shoots Toad into the shaft, the Toad falls through the shaft and breaks the door open, bringing a sapphire with him.