The volcanic eruption in Santorini created a caldera that literally sink the most part of island and created a giant lagoon in the middle. A caldera is formed after large volumes of magma erupt. Then the support to the ground is lost and this leads to the collapse of the surface. In Santorini's case the caldera is filled with sea water. As the magma from the volcano flows to the surface, it builds up to form cones of material. In the center of this lagoon two small islands emerged as a result of this volcanic activity. The islands Kameni and Nea Kameni are only the visible cones while there are many other hidden beneath the sea.
Photo taken at Santorini caldera