The Greek islands are among the most popular holiday destinations in the world, rich in ancient archaeological sites, vibrant local culture and immense natural beauty. Some of the best known are Santorini, Mykonos, Naxos and Ios, which all belong to the Cyclades. Surrounded by crystal-clear waters, each island has its own distinctive beauty and charm. Together they are a small paradise in the heart of the Aegean Sea. The underwater world of the Cyclades is unique and extremely popular for year-round diving excursions and tours. The island of Santorini, in particular, is one of the most famous islands in Greece because of its geology and history of volcanic activity.