Bottom of Well
From the Super Mario Wiki
The Bottom of Well (Bottom of the Well in some regions) is a Treacherous Mansion room in Luigi's Mansion: Dark Moon. Accessed via the well in the mansion's courtyard, it's a simple circular room that leads into a path to the right, ending at the Guard Tunnel.
In the well itself, there is no decoration, but the tunnel next to it has some spiders and a vine that can be pulled to break a brick wall and open the path to the Guard Tunnel.
In Front-Door Key, the Bottom of Well is explored by Luigi in his mission to hunt down the Polterpup that took the titular key.
During Terrifying Invasion, the well can be re-entered like in the first mission, and now has a springboard style flower that can be used to exit again.