The cave of Agios Efstratios is in the eastern part of the island, near Alonitsi beach.

This cave is not particularly known for its geological features, but for its grave historical and religious importance. Here is where Hosios Efstratios the Miraculous found refuge and became a hermit , after being banished and chased during the Iconoclasm in Byzantium. In the prevailing version, the island owes its present name to Hosios Efstratios. Today in the cave you will see icons of Hosios Efstratios and candles that are constantly lit.