Trash Pit

The Trash Pit is an area within Bowser's body in Mario & Luigi: Bowser's Inside Story, and is apparently located in Bowser's Large Intestine. Once Mario, Luigi, Starlow, and all the other people in the meeting at Princess Peach's Castle get inhaled by Bowser, Mario falls at the beginning of the area. Mario then stumbles upon Starlow, who is trapped in a bubble. Once Mario frees Starlow, a Goombule battles him. After the battle, Mario and Starlow proceed into another room, where they find Luigi, attached by his head to a pipe-like object with a bubble. Mario and Starlow free him, and then stumble into ten Attack Pieces, where they learn how to kick a shell at their enemies. Mario and Luigi also learn to use the hammer in this area.

Trivia
This is one of the four places in Bowser's body with Attack Pieces. The others are the Pump Works, Flab Zone and Energy Hold