The Melissani Lake in the Island of Kefalonia, Greece lies at the bottom of a cave whose top caved in giving access to the brackish water table below. I reached it through a man-made tunnel and as the line of tourists moves closer and closer you start seeing the bright turquoise reflection all around the cave and in people´s faces. You then board a small rowboat where a singing and jolly guide takes you over the translucent water and to the underground section of the cave. Nice!