This stage is a unique auto-scrolling level where Mario pilots the Marine Pop, a submarine. The controls change slightly: D-Pad Up/Down moves the submarine vertically, and A/B buttons fire torpedoes. Your goal is to navigate through the underwater caverns, avoiding obstacles and destroying enemies. The primary enemies here are Cheep-Cheeps and Bloopers. Torpedoes are unlimited, so don't hesitate to fire them constantly to clear your path.
There are no traditional power-ups in this stage, but you can collect coins by shooting them or simply touching them. Be careful of the walls and ceilings; colliding with them will damage your submarine. Your submarine has a health bar, and if it's depleted, you lose a life. The stage ends when you reach the rightmost part of the screen. This level is a refreshing change of pace and requires good reflexes for dodging and shooting. Focus on maintaining a clear path and anticipating enemy movements. The Marine Pop is quite agile, so use its maneuverability to your advantage.