Modra špilja, Biševo, Croatia. The Blue Cave, one of the most attractive natural phenomena in the Adriatic, is located on the small island of Biševo near Vis. It is only 17 meters wide and 31 meters long. The cave is accessible through a narrow sea passage between the rocks. Sunlight comes in through an underwater opening, flooding the sea and the visitors with silver and blue colors. The only known habitat for the monk seal is right next to it.