There's an area under the Forest of Earth (at the end of the river) that can only be opened after obtaining the air spirit. What i want to know is what the point of this area is. I don't remember getting anything important there (except for learning the motion for the Dragon Smash technique). In one of the areas I somehow triggered the appearance of a number of zombies; at that point I left the area, and I have not been able to get them to appear again. As far as I can tell, this area is optional. Unfortunately, the only complete walkthrough on GameFAQs is of the "perform these steps in sequence" style, so it's a bit difficult to use it as a reference.