Go to Sylph Cave
In the upper left hand corner of the Underground is a cave surrounded by a grey patch on the ground. Enter the cave. The first thing to do when you enter the cave is to have Rosa cast the Spell "Float" on your whole party (you can do this all at once). This will protect you from the spots on the ground that damage your party and also from monsters that attack using the "Quake" spell.
Inside the Cave
In the first room go straight ahead and into the next room. In the next room you will do the same thing, going to the cave directly ahead of you. In the next room, you may use the save point on the right, then continue to the left, and enter the cave to the left of the one you just entered.
Deep in the Cave
In this long room, head all the way to the end of the room, and through the door there. Now go as far south as possible, and follow the path until you reach a large brown building. Enter this building, and speak to Yang who is in the bed. Now go up the staircase in the building, then step on the platform in the next room. This will take you out of the cave.
Fly to Fabul
Leave the underground, fly to Fabul and enter the castle. Go into the inside of the castle, and up the stairs between the inn and weapon shop. Turn around in the next room and go through the south door.
Get the Pan
Once outside go in the tower on the left, and all the way to the top. Talk to the woman there (Yang's wife) and she will give you the Pan, which you will need later. Leave the castle.
Return to Sylph Cave
Return to the underground, go back in the Sylph cave, and finally return to where Yang was laying. Use the Pan on him, and he will wake up. Since he cannot join you, the Sylphs will give a call spell to Rydia. Go up the staircase, and on the platform again to leave the cave.