From the Super Mario Wiki
As soon as Mario enters the level, he will be greeted by the Noki grandson. The grandson will give Mario the helmet he wore on the last episode and tells him that the Noki elder is waiting at the top of the mountain, where the waterfall begins. Upon meeting the Noki elder at the top, he will tell Mario to dive into the waterfall in order to find the cause of the water's pollution. Once Mario enters the secret cavern under the waterfall, the battle with Eely-Mouth will begin.
Throughout the battle, Eely-Mouth will be spitting poisonous bubbles that will float upward to the surface, damaging Mario if they come into contact. As Mario gets closer, Eely-Mouth will start spinning and will rise up, attempting to eat Mario. With Eely-Mouth's mouth open, the player needs to use the Hover Nozzle to spray Eely-Mouth's teeth, which are covered in poisonous goop. While Mario is spraying, Eely-Mouth will attempt to suck him in and shoot him up to near the surface if he catches him. Once Eely-Mouth is done inhaling, he will close his mouth and sink back down. The player needs to repeat the process until all of Eely-Mouth's teeth are cleaned, which will reward the player with the Shine Sprite.
If the player needs to recover life, there are many Yellow Coins lining the crevices of the cavern.
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