The creation of Santorini is the result of volcanic activities , which last it entire millennium. These volcanic activities created a island with a total area of 103 km2 and according to Herodotus (Greek historian, lived 484 bc to 425bc) this island called strongily (the circular one) due it's shape.
The last high volume explosion of the volcano around 1650 BC made a big part of the island to collapsed into the sea and the water covered the abyss which was created with a initial depth of 800 meters.
Thus the second biggest caldera in the world was created. The collapse must have been followed by huge tsunami that hit the Aegean cost and destroyed the Minoan civilization, and there is a view that influence Egypt.