Santorini is built on the caldera of a long ago exploded volcano. Most of the towns of Santorini are built on top of caldera into the hillsides, overlooking the crater. Santorini is best known for its white washed churches with blue steeples. The island is 96 square km and has 7300 people spread among several small towns. 50% of the pictures you will see in Greek Islands calendars are from Santorini.
As you walk around Ia overlooking the Mediterranean, it is hard to tell when you have seen it all because it meanders so much.