Santorini is an archipelago in the Aegean Sea. It is about 120 kilometres (75 mi) north of Crete. The archipelago is made of five islands, Thira, Thirassia, Aspronissi, Palea, and Nea Kameni in the southern Cyclades. Its volcano is among the few still active in Greece and Europe. These islands formed through intense volcanic activity, with twelve major eruptions happening roughly every 20,000 years. Each eruption caused the central part of the volcano to collapse, creating a large crater called a caldera. Despite this, the volcano repeatedly rebuilt itself.