From the Super Mario Wiki
Pump Works is a location inside Bowser's body in Mario & Luigi: Bowser's Inside Story that seems to be in Bowser's esophagus. This area is accessed to Mario and Luigi when Bowser drinks water from the Sea Pipe Statue in Plack Beach. Once Bowser drinks the water, this area will flood. Once he stops drinking, it will sink. Mario and Luigi travel here to search for Princess Peach, but they find Toadsworth. This area also features Attack Pieces, that once gathered all the ten pieces, Mario and Luigi will learn to use the Fire Flower. Also, it has an obstacle(a cannon that launches food for a giant Chombone) that is required to obtain the Stingler, so they can open up other areas of Bowser's body.
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