Santorini's famous cliffs are the rim of a drowned, still restless volcano
The whitewashed villages tourists photograph on Santorini cling to 300-metre cliffs that are really the inner wall of a collapsed volcano. The sea now fills its crater, about 12 by 7 km and up to 385 m deep, and two dark lava islands in the middle last erupted in 1950.
Santorini, known in antiquity as Thera, sits in the southern Aegean 120 km north of Crete, where the African plate dives beneath the Aegean at up to 5 cm a year. No other stretch of the arc of Aegean volcanoes, which also includes Milos and Nisyros, is as lively, and it counts among the world's sixteen Decade Volcanoes. What looks like a ring of islands, Santorini itself, Therasia and tiny Aspronisi, is what survives of the volcano's rim, with Nea and Palea Kameni, uninhabited and built of lava, rising from the flooded centre.
The present bay is the latest of several collapses. Overlapping shield volcanoes were cut by at least four calderas, the oldest southern one forming about 180,000 years ago and others around 70,000 and 21,000 years ago. The Minoan eruption of the 17th century BC rated 7 on the Volcanic Explosivity Index and may have inspired the Atlantis legend. Yet in 2024 researchers identified an eruption roughly 520,000 years ago that was far larger: its pyroclastic flows reached 70 km from the vent and left deposits up to 150 m thick, more than six times the Minoan volume.
Over the centuries, smaller and mostly gentle eruptions built up the Kameni islands; today only steaming fumaroles remain in their craters. GPS instruments picked up renewed ground deformation in 2011 and 2012. A new phase of unrest began in July 2024, and in early 2025 swarms of earthquakes and rapid uplift inside the caldera prompted the evacuation of 11,000 people. Researchers later traced a mid-2025 swarm to magma moving along a fault northeast of the island, though it stayed more than 8 km down.
The combination of accessible cliffs, sunshine and clear views has made Santorini a magnet for volcanologists as well as holidaymakers. In 2022 the International Union of Geological Sciences listed the caldera among its first 100 geological heritage sites.
Source: Santorini caldera
