Anyone know where this is?

Here's the underground map of BLC. If you look at the upper right part of the map, you can see the edges of another cave. I have no idea what cave this is or how to get there. Anyone know?

This is the second biggest underground area in game, after mushroom cave. I think its part of minotaur cave. As I have never been there I am not sure.

btw, always nice to see 3rd book valley, top right corner

I dont know what cave it is either. Never been to the bottom of Mino lair.