In this area, Mario and everyone else in Princess Peach's Castle become inhaled by Bowser and fall into his body. Mario, Luigi and Starlow apparently landed in this area and had to venture their way out of it.
During gameplay, many Goombules can be found here to get in the bros. way. Hammers can also be obtained near the end of this area as there are crates blocking the area and Mario and Luigi need to break them with their hammers. Ten Attack Pieces must be obtained here to gain the Green Shell attack.
Mario also had to free Luigi and Starlow who were in trouble as Starlow and Luigi had their heads trapped in bubbles and Luigi's in a pipe-shaped one.