Expanding on Chase36535's answer, as I've just come here looking for the answer: Yes, it's part of the Sorcery mod. As you enter Western Pass, go over the two rope bridges, and then follow the narrow path along the canyon wall. There's a set of rope and post fences. Just past the second set, and before you enter into the canyon walls, look down to your right, there's a cave that you can drop down to. That's White Tooth Cave. (Also worth noting that if you jump in the river, and let it carry you along, you'll end up somewhere else as well.)
