It is the largest cave in the Cyclades. As the speleologists say, nature has created a synthesis of stalactites, stalagmites, and the rare cave phenomenon of speleogala (moonmilk).
According to the locals, a shepherd discovered the cave, and in it the icon of St. John from whom it takes its name. On August 28 a festival in his honor takes place in the area, just after the religious ceremony conducted in the cave. This celebration also welcomes visitors from the surrounding islands. A path from the village of Panagia will lead you to the cave, and the spectacle within. The festival, and the view of the sea, will compensate you for the walk. The legend also connects this cave with the mythical Cyclops Polyphemus, who lived in it and was blinded by Odysseus. During Odysseus’ flight, the blinded Cyclops threw rocks which, according to legend, created the Avelonisia islets.