Bottom of the Well
From the Super Mario Wiki
The Bottom of the Well (Japanese: 井戸の下 Bottom of the Well) is the smallest location of the mansion in Luigi's Mansion, though it technically is not part of the mansion itself. It is cleared in the third Area.
In this location, Luigi peers through the mouth of a stone lion into a small room, where he sees Mario trapped in a painting in the Secret Altar. After Luigi looks through the lion, a Ceiling Surprise (a Purple Bomber in the PAL Hidden Mansion) attacks. After Luigi captures the ghost, the Key to the Rec Room appears.