Under the Stairs is a room in Gloomy Manor in the game Luigi's Mansion: Dark Moon. This room can be accessed from the Patio and has a door leading behind the Fish Tank in the Foyer. It is one of the smallest rooms in Gloomy Manor.
Luigi can get into Under the Stairs once he acquires the Dark-Light Device. In A-5: Sticky Situation, Luigi can head out to the Patio and reveal a hidden door that leads to Under the Stairs. From there, Luigi can enter through another door to the Foyer to get the Gem that was hidden behind the fishtank.