Everything that the light does not touch is the Unterzee.
Ok, in all seriousness, why is it called the Unterzee?
Well you all know what the zee is but what about the "Unter"? Is it because it's the "Other" sea, as in a dark version of the common sea? Or is the meaning "Under" like underground sea?
Whatever you fancy, the Unterzee is the main setting in Sunless Sea, though travel to the surface is possible, yet limited.