FFV - Walkthrough - Gill's Cave
You'll come to a room with four chests. Open the chest in the middle to find a big stone. Put the stone into the upper-left chest. A door will open and you can go through to use a switch. Go back to the room with the chests, take out the stone of the box and put it into the lower-left chest. Go through the door and follow the path, which will be invisible until you get closer to it. Press the switch you find there and a staircase will appear. Go down the stairs and you'll have to follow some invisible paths again. As soon as you find a pond, jump into it. You will find Sage Gill now. He will tell you to visit the Moore Forest. Leave the cave and enter the submarine. Navigate to the blinking point in the south-west of the worldmap. Dive up and you'll be on lake east of the Moore Village. Get out of the submarine and visit the Moore Village.