As I had gotten through the hole, I saw this sight. It's a bit of a fairy tale: you walk a thorny path, pass through the hole in the mountain and you are met by quiet and beautiful lake. (Which is likely to rather be a dam; in the walking map I later got hold of it was referred to as a barrage, but I could not see any power plant. Nor the walls of any dam; the outlet from the lake was hidden behind the rocks.)