Flame Pipe
The Flame Pipe is one of the areas in Bowser's Body, most likely located somewhere in Bowser's lungs.

When Bowser sees some flimsy material blocking a passage in Cavi Cape, he tries to breathe fire, but he cannot. After a few more attempts at fire breathing, he gives up and is confused as to why he can't.

Later in the game, Mario, Luigi, and Starlow encounter a giant Scutlet sitting on top of the bottom part of the pipe. It being there has caused the fire traveling through the pipe to be obstructed, preventing Bowser from using his fire breathing ability. The giant Scutlet stick his tongue out at Starlow and sucks her into his body and Mario and Luigi must fight him to retrieve Starlow back. When Mario and Luigi defeat the Scutlet, they obtain Starlow back and Bowser gains the ability to breathe fire again, allowing him to burn down trees and other flammable objects.