The caldera crossed by Santorini's sailing fleet is the drowned throat of a volcano that lost roughly 60 cubic kilometres of itself in a matter of days. Around 1600 BCE, the Minoan eruption emptied the magma chamber beneath ancient Thera, and the island's centre collapsed into the sea.
The best Santorini catamaran cruise tours now cross that void — a flooded basin some 12 kilometres long and 7 kilometres wide, with water reaching 390 metres deep. The cliffs above read as a cross-section: dark lava at the waterline, pale pumice higher up, a white ash cap laid down at the end.
Beneath that ash lies Akrotiri, a Bronze Age town of two- and three-storey houses with paved streets and clay drainage pipes, excavated systematically from 1967 under Spyridon Marinatos. No human remains have been found there. The population appears to have read the warning tremors and left. Frescoes of saffron gatherers and blue monkeys survived under twenty metres of pumice. Ash from the same eruption is still used by archaeologists to date sites from Crete to the Nile Delta.
The volcano was not finished. Nea Kameni, the black islet at the centre of the caldera, grew in stages between 1707 and 1711, again from 1866 to 1870, and last erupted in 1950. Sulphur springs off neighbouring Palea Kameni stain the shallows a rust orange and warm the water by several degrees. Holders of best Santorini catamaran cruise tickets are, in effect, given a seat above a live vent: the seabed here still rises and falls by centimetres each decade, tracked by GPS stations set along the rim.
Vlychada, on the south coast, came late to all of this. The bay served a tomato-processing plant built in 1945, when the island exported paste rather than passengers; the factory closed in 1981 and its stone shell now holds a museum. Wind has cut the pumice behind the marina into pale ribs and hollows that look carved rather than eroded. For most of the island's history there was no harbour worth the name — cargo was landed at Skala Fira and hauled up the cliff by mule. Today a Santorini caldera sailing tour leaves from a dredged, breakwatered basin barely older than living memory, and the line a best Santorini catamaran cruise tour draws past the Red Beach, the White Beach and Thirassia is a reading of that same geology from the water.
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