Santorini Caldera

The Santorini caldera is a flooded volcanic basin about 12 by 7 kilometres across, with water reaching a maximum depth of 385 metres and cliffs rising some 300 metres above the sea on three sides. Five islands ring or sit inside it: Santorini itself (Thera), Therasia, the uninhabited islet of Aspronisi, and the two lava islands of Palea Kameni and Nea Kameni at the centre. This page covers how the basin formed, what the volcano is doing now, and what there is to see around the rim.

How the Caldera Formed

Before the collapse, the island was known as Strongyli, "the round one" in Greek, after its shape. The ground here has failed repeatedly: volcanic activity in this part of the southern Aegean has produced several successive calderas over roughly the past 180,000 years, and the one visible today is only the most recent.

The basin took its present form in the Minoan eruption of Thera, dated to around 1610 BC, give or take a few decades. Radiocarbon dating points to the late seventeenth or sixteenth century BC, while dating based on Egyptian chronology has argued for a later window, and the two methods have not been fully reconciled. It was a Plinian eruption with a Volcanic Explosivity Index of 7, throwing out an estimated 28 to 41 cubic kilometres of dense-rock equivalent material through a column some 30 to 35 kilometres high. The collapse that followed generated tsunamis that struck the north coast of Crete, 110 kilometres away.

The eruption buried the Bronze Age town of Akrotiri on Thera under metres of ash and destroyed Minoan coastal settlements. No human remains have been found at Akrotiri, which suggests the inhabitants were warned by earthquakes and left before the eruption. It is often said that the eruption ended Minoan civilisation outright; that overstates it. Minoan Crete carried on for more than a century afterwards, though the damage is generally counted among the causes of its decline.

The Kameni Islands and the Live Volcano

Palea Kameni and Nea Kameni, the "old" and "new" burnt islands, were built up by a long series of smaller eruptions at the centre of the caldera after the Minoan event, most of them submarine at first. Nea Kameni carries the active craters, reached by a marked path from the landing stage, and the rock underfoot is still warm in places with sulphur staining around the vents.

The volcano is not extinct. The last eruption ran from January to February 1950. GPS monitoring recorded ground deformation around the caldera in 2011 and 2012, and a period of intense seismic activity affected the area between mid-2024 and early 2025. Kolumbo, a separate submarine volcano about seven kilometres northeast of the island, erupted in 1650.

Warm, iron-rich springs seep into the shallow bay on the side of Palea Kameni, staining the water orange-brown, and swimmers anchor off and swim in. Because the caldera is deep almost to the foot of the cliffs, boats here moor in open water rather than alongside a beach, and this bay is one of the anchorages you'll visit on a private gulet cruise in Greece.

Around the Rim

Four places on the rim account for most of what visitors come to see:

• Fira, the island's capital, sitting directly above the caldera wall.
• Oia, on the northwestern tip, known for its sunset view over the basin.
• Imerovigli, the highest of the rim villages, built on the ridge below the old Skaros rock.
• The Akrotiri archaeological site in the south, where streets, houses and wall paintings survive under the ash of the Minoan eruption.

The footpath along the rim from Fira to Oia is the main walk on the island, roughly 10 kilometres and three to four hours, running past Firostefani and Imerovigli with the caldera open on the left the whole way.

Staying and Eating on the Rim

Cliffside hotels concentrate in Oia and Imerovigli. The traditional building type here is the yposkafo, an arched dwelling tunnelled into the soft pumice; the rock holds a stable temperature and stays cool through the summer, which is why the form survives in converted hotel rooms.

The volcanic soil and the near-absence of rain in summer produce a distinctive local harvest: small, intensely flavoured cherry tomatoes, white aubergines, capers and fava, which on Santorini is made not from broad beans but from a local pulse, Lathyrus clymenum. The island's white wines come mainly from the Assyrtiko grape, grown in low basket-trained vines that shelter the fruit from the wind, along with the sweet sun-dried Vinsanto.