When the volcano erupted creating the island of Santorini (and other Cycladic islands) the eruption created a "caldera" in the center of the islands.
This image shows the caldera as a background for one of the many domed churches on Santorini.
The caldera is also the area where tourist ships come into port at the base of the island, about 500 feet below the city of Fira.
One of those ships is just coming in and can be seen in the background on the left of the dome.