Formed by the massive volcanic explosion that blew the center out of the island some 3,600 years ago, the calder is the sea-filled volcanic crater that remained.
Measuring 12 kilometers by seven kilometers, it is still home to volcanic activity - in its center rise the two Kaiméni islets with hot springs and gas emissions.
Various agencies offer one-day excursions of the caldera by boat, including time to bathe in the hot springs and then have lunch on Thirassia, a tiny island on the west side of the caldera affording amazing views back to Santorini across the water.
