The Bottom of Well is a room found on the outskirts of the Treacherous Mansion. It is first reached by Luigi during the events of the level "E-1: Front-Door Key", and grants him to get to the underground section of the area.
Almost at the beginning of "E-1: Front-Door Key", Luigi tries to open the front door of the mansion with the key given him by E. Gadd, but fails due to Polterpup that springs out casually from the front door and takes the key away. In order to proceed, Luigi needs to go to the left area out of the Treacherous Mansion, where he will find a well. By using the Dark-Light Device, he will uncover the well's rope, and then be able to get to this room by using it.