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. It is, as implied, full of miscellaneous junk. When Mario, Luigi, Starlow, and all the other people in the meeting at Princess Peach's Castle get inhaled by Bowser, Mario falls into the beginning of this area. Mario then stumbles upon Starlow, who is trapped in a amoeba. 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. They free him, and then stumble into ten Attack Pieces, and Mario and Luigi learn how to kick a shell at their enemies. Mario and Luigi also learn to use the hammer in this area. Although only Mario can use it at the time.

Trivia

 * This is one of the four places in Bowser's body that contains Attack Pieces. The others are the Pump Works, Flab Zone and Energy Hold.
 * Despite being in Bowser's large intestine, The Trash Pit is clearly in his armpit on the map of his body.