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I don't really remember it was long ago and I don't seem to have a save around that time, but as the story unfolds with Dulrig, Rueben and the Emerine crystal I think those are your best bet, turned out Rueben visited the caves before you did

If I remember correctly, you just have to walk past the wall there and there's a chance your character notices it. It may be an invisible perception check, but try going past there a few times and you should get the dialogue that opens it up! I remember unlocking it by accident before the story with Dulrig, Rueben, and the Emerine Crystal, but if it still doesn't work then try progressing story a bit more and going back again.