The Sewer is an area found in the Haunted Towers of Luigi's Mansion 2. It can be first reached optionally by Luigi during the events of "B-2: The Pinwheel Gate", if he wants to capture that mission's Boo, or mandatorily in "B-3: Graveyeard Shift", after beating The Three Sisters in the Old Graveyeard and rescuing the Toad assistant.
The Sewer is a small, narrow area similar to a hallway. Here are found many arches along with red plants hanging from its ceiling. To the far left, near the end of the room, Luigi can yank off a moss covering with mushrooms to reveal another smaller area, wich contains just two coffins (one that is a teleport) and many candles. The Boo is found by pulling off the first red plant found on the ceiling after entering the room (only on mission B-2).