Blue Grottoes, as the name suggests, refer to those blue caves that automatically emit blue light. However, I almost missed seeing them. The Greek sea is a beautiful, azure blue. However, you have to take a speedboat to find a deeper blue in this azure sea. There is such a place at the northernmost corner of Zakynthos Island. Turn into the cave and look down carefully through the transparent deck at the bottom of the boat. The bright blue is deeply imprinted in my mind...