When you first start out in Bonfire Island, it sort has a yin-yang type formation that leads up to a den/rock ledge. Below that rock ledge, on the main, larger (and lower) portion of the map is a black hole on the side of the mountain. Once in there, you arrive in Ficho tunnel in an area with a few rocks and boulders that is pretty wide. Just follow that tunnel to the first bend to the left. On the right should be a roughly circular pool with a waterfall which can be used to access the Lonely Cave, South Pole, Ascension Land, etc. If you keep following the path to the left of the waterfall-pool area, you come across another waterfall, leading down to the main area of the Ficho tunnel. The pool below it slopes upwards into a bank on both sides with a small river running between them. If you follow that a ways, there's another pool with a crystal in it. Just keep going until the final bend to the right which slopes upward (stay on the right side and jump over the steep part of the bend). At the top is the portal to Fluorite Plains, Temple of Dreams, etc.
If you need any more help, just ask. :>
