Hmmm underground tunnel network, thats below lirs reach,,,, soumds a lot like crookbacl to me :/I'm not sure but a while ago there was a chat about a huge tunnel system below Lir's reach. If I'm right they are working on it ATM. So maybe that spot behind the waterfall could be a secret entrance to a spot in the tunnel system you normally can't enter when you are already inside the tunnels.
Orrr is it a whole new place