I have been exploring a fair bit in the year or so that the pre-order has been out. Not once have I seen any kind of tunnel or proper cave system apart from the small shallow caves.
This game would really benefit from caves that lead to tunnels that lead to caverns where tribes might live in or some other NPC type animal might dwell.
Imagine the excitement and depth it would give to the game for a group of players to enter into a uncharted cave for the first time and for there to be a need for torches for light. Spooky echo effect on the sound could be added.
Perhaps a tunnels system that goes right through these mountains. Tribes could use them to navigate around the mountains and appear at the other side of the range.
There might be rare deposits in the tunnels - an old mining system.
I do hope that this is considered as a way of expansion before new lands are laid out. I think it is a real shame this is not in the game already though as adding them later would be a bit strange. Perhaps one of the new land areas might have tunnels and caves.