Aegean Sea

Long ago, Santorini was a pointed volcano.

Then big eruptions emptied the hot rock under it, and the middle fell in. The sea filled the hole.

NASA's Earth Observatory says Santorini once had the typical cone shape of a volcano with steep sides. Over hundreds of thousands of years, a series of eruptions emptied the magma chamber under it, and it collapsed, making a bowl-shaped hollow called a caldera.

About 3,600 years ago, a very powerful eruption threw ash tens of kilometers into the air and left the caldera hundreds of meters deep and partly flooded by the Aegean Sea. The nearby islands of Thirasia and Aspronisi are what is left of older eruptions, including ones 180,000, 70,000 and 21,000 years ago.

According to NASA's Earth Observatory, eruptions over hundreds of thousands of years emptied Santorini's magma chamber, which eventually collapsed to form a caldera. Before the great eruption, the island had been built up by layers of lava from overlapping shield volcanoes and had already seen three significant caldera-forming eruptions; the fourth and latest major eruption created today's islands and caldera bay.

That eruption, about 3,600 years ago (dated in NASA captions to around 1620 or 1640 BC), hurled vast amounts of ash tens of kilometers high and left the caldera hundreds of meters deep and partly flooded. Thirasia and the smaller Aspronisi are remnants of older deposits and overlapping calderas from eruptions about 180,000, 70,000 and 21,000 years ago.

In the middle of the flooded caldera there are two small, young islands.

They were made by lava that came out much later.

In the center of Santorini's caldera sit two small islands, Nea Kameni and Palea Kameni. They are much younger than the rest. They grew from lava domes and lava flows in gentler eruptions that began about 2,000 years ago.

NASA noted in 2008 that the most recent volcanic activity on the Kameni islands was in 1950, with some small explosions and new lava. Around Santorini, there are also hot springs on the sea floor called hydrothermal vents.

Nea Kameni and Palea Kameni, in the middle of the caldera, formed from lava domes and flows during a less explosive phase that began about 2,000 years ago, NASA's Earth Observatory reports; several dark lava flows can be seen from space. The most recent volcanic activity in the Kameni islands, as of NASA's 2008 caption, was in 1950, including some small explosions and lava production.

The complex underwater terrain and hydrothermal vents around Santorini and neighboring underwater volcanoes have also drawn scientists testing robotic submersibles, as practice for exploring distant worlds.

The great eruption covered an old town with ash.

The ash kept its houses, roads and paintings safe for thousands of years. Now people dig to learn about it.

The eruption about 3,600 years ago buried the Bronze Age town of Akrotiri under a layer of ash and volcanic rock several meters thick. That layer kept the town in amazing detail: buildings several stories high, roads, water channels for crops, furniture, pottery and wall paintings.

Archaeologists have been digging at Akrotiri since 1967. Today, lines of white buildings, including many hotels and guesthouses, sit along the caldera rim.

Archaeological evidence indicates the eruption devastated what had been a thriving Bronze Age settlement. Yet the ash and tephra it laid down, several meters thick, buried and preserved the Minoan town of Akrotiri in exquisite detail, with multi-story buildings, roads, irrigation systems, furniture, pottery and frescoes; excavation has continued since 1967.

Today, white-roofed buildings, including many hotels and guesthouses serving the millions of travelers who visit each year, line the caldera rim. NASA's 2008 caption notes that volcanic activity is closely monitored by the Institute for the Study and Monitoring of the Santorini Volcano (ISMOSAV).

Scientists keep a close watch on Santorini's volcano.

At the beach, grown-ups help everyone swim safely.

NASA noted in 2008 that Santorini's volcano is closely watched by a monitoring institute, ISMOSAV, so it is wise to follow local advice on any visit.

When swimming in the sea anywhere, know what to do in a rip current, a powerful, narrow channel of fast-moving water. NOAA's advice: don't fight it. Swim parallel to the shore, then swim back to land at an angle.

NASA reported in 2008 that volcanic activity at Santorini is closely monitored by ISMOSAV, the Institute for the Study and Monitoring of the Santorini Volcano; following local advice is PlayOwl's tip built on that monitoring.

For sea swimming, NOAA's National Ocean Service describes rip currents as powerful, narrow channels of fast-moving water that can move at up to about 2.4 m/s (8 feet per second), faster than an Olympic swimmer. Swimming straight back against one exhausts swimmers; instead, swim parallel to the shore and return to land at an angle.