From the Super Mario Wiki
The Basin is a room inside of the Secret Mine in Luigi's Mansion: Dark Moon. The Basin is accessed by taking the bottom right door inside of the Airway or by taking the northwest door inside of the Drift Hall. The Basin is a large room with the entire floor covered in ice. This ice shows the reflection of various objects, even ghosts and items hidden by Spirit Balls. There is an area to the north boarded off and several crates of explosives scattered around. The door on the west leads into the Airway and the door on the south leads into the Drift Hall.
During D-1: Cold Case, Luigi needs to go through the Basin to reach the Airway. However, when he enters the Basin, he has to fight three Sneakers. Once the Sneakers are defeated, Luigi can progress. On the northeast side of the room, there is a Robomb hidden by Spirit Balls, but its reflection can be seen on the ice below. After using the Dark-Light Device to reveal it and detonate it, three gold frogs pop out. In D-3: Across the Chasm, the Strong Gobber escapes into the Basin. Before fighting the Strong Gobber, Luigi must reveal a missing Robomb with the Dark-Light. After detonating it, Luigi has to fight three Hiders before ultimately fighting the Strong Gobber.