Mt. Deeprock

Certainly not the tallest mountain, it is, however, the deepest, in the sense that it has many cave networks and tunnels formed from many generations of ground and rock pokemon digging their way through. Rather then having a pointed top, Mt. Deeprock's summit rounds off into an almost plateau like surface, and can sometimes be home to many flying pokemon. There are also rumoured to be a few underground lakes in the deepest parts of the caverns.