Indeed, there is: You can toogle the fog of war like so: Options>Video>Performance>On/Off.
Hmm ... it's strange, I was Always thinking the fog of war is set ON by me, but it wasn't ! But set in ON and here & there now I see "fog", but the areas I haven't visited yet are like always "black". So I was always thinking the "black" undiscovered areas = fog of war, but I'm wrong !
