Corona Mountain is an active volcano which looms over Delfino Plaza. While the hot springs at the base of the mountain are a pleasant attraction for tourists, the lava caves within are deadly. Mario can gain access to Corona Mountain from the area behind the Shine Gate in Delfino Plaza after he has completed episode 7 of every location on the game. Unlike the other areas which stretch off of the plaza, Corona Mountain does not consist of multiple episodes, but instead involves a platforming section leading up to the final boss fight of the game.
Mario starts the level with a long tunnel ahead of him, filled with lava and two types of platform; one covered in spikes which periodically raise and lower, and one constantly burning with flame. Mario must move over the spike platforms by jumping on and off of them with the right timing, and squirt the fire platforms with water to extinguish their flames. Lava Bubbas may also occasionally pop up from the lava, which Mario can either avoid, jump on, or spray with water to defeat.
After the platforming section Mario must get into a boat and use FLUDD to propel himself to the centre of the mountain. Scattered throughout the lava are small blocks which spray out water which Mario can use to refill FLUDD, however, should the boat collide with anything it will sink into the lava. Once Mario reaches the central area he must then grab a rocket nozzle and use it to make his way up a series of ascending platforms to the final area.
The final area of the game is a giant hot tub full of green slime that Bowser and his son are relaxing in. Princess Peach is also present there, sitting on a giant rubber duck. In the final fight Mario must dodge the attacks of Bowser and his son, and ground pound the three platforms which exist at the tips of arms which lead off of the hot tub. After all of these have been destroyed, the hot tub will tip over and the game ending will begin.