Caldera is the terrestrial cavity formed when a part of a volcanic cone collapses or when its inner walls are gradually corroded. The collapse is triggered by the emptying of the magma chamber beneath the volcano, usually as the result of a large volcanic eruption. If enough magma is erupted, the emptied chamber will not be able to support the weight of the volcanic edifice (the mountain) above. Fractures will form around the edge of the chamber, usually in a roughly circular shape. These ring fractures may in fact serve as volcanic vents. As the magma chamber empties, the center of the volcano within the ring fractures begins to collapse. The collapse may occur as the result of a single massive eruption, or it may occur in stages as the result of a series of eruptions. The total area that collapses may be hundreds or thousands of square kilometers.

The Minoan eruption of 1630 B.C. resulted to the formation of Santorini's caldera, which is actually a ring of islands formed by Thera, Thirasia and Aspronisi; this marine basin, or caldera, has a surface area of 84.5 Km2. After that catastrophic eruption, the eruptive activity of Santorini was by no means over. For the last 2000 years, the enormous caldera that had been enlarged and deepened by the eruption of 1640 BC has been the scene of renewed volcanism: the caldera is filling again and the volcanic edifice is changing its shape. In that way, the islands of Nea and Palea Kameni were formed in the centre of the today's caldera. The greatest depth is in the northwest basin, which reaches nearly 400 meters below sea level.

source: Fire in the sea, Walter Friedrich