From the journal

How Santorini's caldera was made

8 min read

Santorini's caldera is the hole left when a volcano erupted around 1600 BC and its emptied roof collapsed into the sea. That is how the basin formed: about 11 km across, roughly 390 m at its deepest, ringed by three surviving fragments of the old island, with two new ones growing back in the middle.

A caldera is a cauldron. The word comes from the Latin for a cooking pot, and it means a volcano that fell into its own empty chamber rather than one that built a cone.

This is written for someone who wants the actual mechanism and does not want to wade through jargon to get it. Every technical word below gets translated the first time it appears.

You are inside the volcano, not next to it

This is the part that takes a minute to land.

The cliffs on your left as you sail north are not a coastline that happens to be steep. They are the inside wall of a volcano. The water under the keel is the crater floor, flooded. The white villages on the top edge are standing on the rim.

Once you have that, the whole shape of the island explains itself. Santorini is a crescent because a circle lost its middle. Thirassia is the piece that ended up on the far side. Aspronisi is a chip of the same ring. The gaps between them, north-west and south-west, are where the sea broke in — and they are the gaps every boat uses to get in and out.

How it works

What happened, in order

Four stages. The whole thing, from the first ash to the collapse, is thought to have taken days rather than years.

1. There was a round island with a mountain on itBefore the eruption, Thera was a more or less continuous ring of land — the shape survives in the three fragments that are left. Underneath it sat a chamber of molten rock. Akrotiri, the Bronze Age town on the southern arm, was a going concern with drains and two-storey houses.
2. The eruption emptied the chamberThe column of ash and pumice is estimated at 30 to 35 km high — roughly three times the cruising altitude of an airliner. Volcanologists rate it VEI 7 on the explosivity index, a scale that runs 0 to 8, where each step up is roughly ten times the material. Ejecta, meaning everything thrown out, is put at 28 to 41 km³ of solid rock equivalent.
3. The roof fell inWith the chamber empty there was nothing holding the mountain up. It collapsed into the space beneath it. That collapse, not the explosion, is what made the basin — a caldera is a subsidence feature, not a blast crater. The sea poured into the hole through the broken rim.
4. The sea moved, hardThe collapse generated tsunamis put at 35 to 150 m. Ash from the eruption is found across the eastern Mediterranean and in ice cores, which is how it can be dated at all. Akrotiri was buried under metres of pumice, and it is still being dug out.

The numbers

The caldera in numbers

Figures from the Santorini caldera and Minoan eruption entries at en.wikipedia.org and from greeka.com, seen 25 August 2026. Where sources disagree, both figures are shown.

	Figure	In plain English
Age	About 3,600 years. The eruption is dated around 1600 BC.	Younger than the pyramids at Giza. Older than the Parthenon by a thousand years.
Size of the basin	About 11 km across; published as roughly 12 km by 7 km.	A boat crossing it at 12 knots takes the better part of half an hour.
Deepest water	Roughly 390 m. The published maximum is 385 m.	Deeper than the cliffs are tall. This is why nothing anchors in the middle.
Cliff height	Up to about 300 m; around 260 m at Fira, around 350 m at Imerovigli.	The rim is not level. Imerovigli is the high point of it.
Islands on the rim	Three: Thera 75.8 km², Thirassia 9.3 km², Aspronisi 0.1 km².	All three are leftovers of the island that was there before.
Islands inside	Two: Nea Kameni 3.4 km², Palea Kameni 0.5 km².	These are new. They grew back up inside the hole after the collapse.
Explosivity	VEI 7.	The Volcanic Explosivity Index runs 0 to 8. Each step is roughly ten times the material of the one below.
Material thrown out	28 to 41 km³ dense-rock equivalent.	Dense-rock equivalent means the volume if you squeezed all the ash and pumice back into solid rock, so the number is comparable between eruptions.
Eruption column	30 to 35 km high.	About three times the altitude of a passenger jet.
Tsunami	35 to 150 m.	The range is wide because it is reconstructed from deposits, not measured.
Earlier collapses	Roughly 180,000, 70,000 and 21,000 years ago.	This is the fourth caldera on this site. The volcano has done this before.

The dating is genuinely contested. Radiocarbon puts the eruption at 1619 to 1596 BC on the most probable range; conventional archaeological dating gives roughly 1550 to 1480 BC. That is a gap of about a century, and it is not settled.

Reading the cliff from the water

The cliff is a cross-section, and once you know what the bands mean you cannot stop reading them.

The pale cap along the top — white to cream, often several metres thick — is tuff: ash and pumice from the Minoan eruption, welded together over time. That layer is the eruption itself. Everything beneath it is older.

The red, brown and black bands below are earlier lava flows and earlier ash falls, stacked in the order they arrived. Red usually means iron that has oxidised, the same chemistry that colours Red Beach on the south coast.

Aspronisi is the clearest single lesson on the whole route. It is a small uninhabited fragment of the pre-eruption island, and it sits in the south-western gap. Its base is dark lava rock. Above that sit thick pale layers of pumice and ash. Black is before, white is the eruption. Nothing else on the cruise puts it that plainly.

The two islands in the middle of the basin are a separate and much more recent story. We followed that one in the volcano from the water.

Why the villages sit on the rim

Fira, Firostefani, Imerovigli and Oia run along the top edge of the cliff in a line, and Manolas faces them from Thirassia across the water. None of that is an aesthetic decision made for the view.

The rim is the high ground, and it is what survived. Its inner face is a 200 to 300 m wall that no one is climbing uninvited, so a village on the lip is defended on one side for free. Imerovigli sits at the highest point of the rim, roughly 350 m, and its name comes from its job: the daytime lookout post, watching for pirates. Skaros, the rock below it, carried the island's medieval capital and a Venetian castle.

The other half of the answer is the sea. There is no shelving shore inside the caldera and almost nowhere to beach a boat, so the caldera side was never where you lived to fish. It was where you watched from. The beaches and the farmland are on the outside of the crescent, facing east and south, which is exactly where the modern resorts ended up.

The Atlantis question

It comes up on every cruise, so here it is briefly and honestly. Plato described Atlantis in two dialogues written around 360 BC, roughly twelve centuries after this eruption. Some scholars have proposed that a folk memory of the Minoan collapse fed into that story.

It is a hypothesis. It is not established, and it is not testable with anything currently in the ground. The real Bronze Age town is at Akrotiri, it is spectacular, and it does not need a second name to be interesting.

What is still argued about

The date, by about a century. The volume, which has been revised upward as marine deposits were surveyed. And whether the largest ships can anchor in the caldera at all — sources disagree, which is why we describe the mechanism instead. The floor is 300 to 400 m down and the walls fall away almost vertically from the shoreline. That is beyond normal anchoring scope, and it is why cruise ships lie off and tender their passengers ashore.

Ready when you are

See it from inside it

Five hours, 10:00 to 15:00, €100 per person, fifteen guests at most. Send us a date and we will tell you what is free.

Check availability & book

Call the harbour office+30 698 977 1888

Common questions

How was the Santorini caldera formed?

By collapse, not by explosion. Around 1600 BC an eruption emptied the magma chamber under the island, the roof above it had nothing left to rest on, and it subsided into the void. The sea then flooded the hole through gaps in the broken rim. The basin is the shape of what fell in.

How deep is the Santorini caldera?

Roughly 390 m at its deepest, with 385 m published as the maximum. The floor sits 300 to 400 m down across most of the basin and the walls drop almost vertically from the shoreline. That is why cruise ships lie off and tender passengers ashore rather than tying up alongside.

What is Aspronisi?

The smallest of the three rim islands, 0.1 km² and uninhabited, sitting in the south-western gap between Akrotiri and Thirassia. Its name means white island. Dark lava at the base, thick pale eruption deposits above: it is the clearest before-and-after section visible anywhere on the caldera route.

Is the Santorini volcano still active?

Yes, and it is monitored continuously. Nine eruptions have been recorded since 197 BC, building the two Kameni islands in the middle of the basin. The most recent was in 1950. The volcano is quiescent now, which is the technical way of saying quiet rather than finished.

Why can't boats anchor in the middle of the caldera?

Depth. The floor lies 300 to 400 m below the surface and the walls fall away almost vertically, so there is nothing for an anchor to reach on any normal length of chain. Boats work the edges instead, and the swimming stops are all in shallow water close to a shore.

Was Santorini the real Atlantis?

Nobody knows, and we would rather say so. Plato wrote his account around 360 BC, about twelve hundred years after the eruption, and the link is a scholarly proposal rather than a finding. The Bronze Age town at Akrotiri is real, buried by this eruption, and worth the visit on its own terms.

Santorini Boat Tour - Water Taxi is a catamaran cruise and water taxi operator based in Fira, Santorini.

Updated August 2026