Santorini is what remains of a colossal volcano explosion over 2,000 years ago. Quite possibly the largest in the history of the planet. What you see here effectively are the parts of the volcano remaining above sea level. The main populated island forms something more than a semicircle of rock rising steeply from the volcano floor. The cliffs reach 1,000 feet above sea level and under the water descends to 1,000 feet deep before rising back above sea level to form a cone in the center of the circle that would exist if the semicircle could be completed into a circle, (imagine a circle with an inner solid center). Starting at the right of the picture you see multicolored cliffs rising, with a town precariously perched 1,000 feet above the water. This is Fira, the main settlement. You can see the zig zag path leading down to the water, and if you look carefully you can also see the cable car leading up. Continuing around further away you can see the village of Oia, beautiful and also perched high above the water.

Toward the left side of the picture you see an island close in, this is the volcanic core (the center of the "lagoon"), still active, and moving to the left of that os the last stretch of the semicircle on which Oia and Fira are perched (some of the semicircle is behind the camera).