Pump Works is a location inside of Bowser's body that is in the game, Mario & Luigi: Bowser's Inside Story. It is located right above Toad Square and right below the Airway. Pump Works also appears to be inside Bowser's throat.
Mario and Luigi can access the Pump Works once after Bowser defeats the Sea Pipe Statue. As Bowser keeps drinking the water that is spraying out of the statue, Mario and Luigi can swim up to higher areas in Pump Works in order to process through the game. At some points, Bowser needs to stop drinking water in order for the Mario Bros. to get past certain obstacles. Mario and Luigi venture here to find Princess Peach but they end up running into Toadsworth who is also looking for the Princess. They all leave the Pump Works after that.
There also also ten Attack Pieces in this area. If Mario and Luigi collects all of them, then they can learn the Fire Flower move.