The volcanic caldera showing up quite well here.
'Twas the site of one of the largest eruptions in recorded history, apparently - the Minoan Eruption. Prior to this, the ring of islands had been almost complete.
The volcano is dormant, but there is a little activity at the island in the middle, which is named Nea Kameni - it first made it's appearance in the Roman era and was noted by people writing at the time.
For a sense of scale, you can see some cruise ships at the far side of the caldera.
