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[edit] Entering the Mountain

Once you have every Shine in every world up to level 7, the plaza will be flooded. Head to the area where you use the rocket nozzle to get to the area behind the Shine Gate (there is no point in collecting the rocket nozzle though). There is a hole in the mountain that used to be closed up. Remember?Well, there is your entrance. Go in to enter the volcano.

[edit] Crossing the magma river

Hop into the boat. To steer, shoot in the opposite direction to that in which you want to go.