Basement (Luigi's Mansion 3)
From the Super Mario Wiki
The Basement is an area in Luigi's Mansion 3. It is located below the Grand Lobby. It contains a laundry room and parking garage, as well as Professor E. Gadd's lab. Luigi first explores this area near the start of the game, when he falls through the laundry chute after being chased by King Boo down the hallway of RIP Suites. He falls into the laundry room and is awoken by Polterpup, who leads him to the parking garage where he finds a red vehicle with the Poltergust G-00 inside. Polterpup then teaches Luigi how to use the vacuum, as well as the Strobulb and Burst. Luigi is then able to walk up to the first floor of The Last Resort.
Luigi returns to the basement later on after rescuing Professor E. Gadd. The professor sets up his portable laboratory and gives Luigi the Virtual Boo, allowing them to communicate at any time. He then sends Luigi on a mission to get his briefcase from the fifth floor. When Luigi returns with the briefcase, Professor E. Gadd opens it to reveal a capsule with a green goo-like substance which he injects into the Poltergust G-00. He then teaches Luigi how to use Gooigi. Luigi can return to Professor E. Gadd's lab at any time to play ScareScraper mode, view collected ghosts and gems in the vault, and talk to the professor.