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The Stefanos volcano crater on the island of Nisyros in GreeceWe began our exploration of Nisyros, during our gulet voyage, by visiting Palaiokastro, one of the finest Ancient Greek fortifications to survive (and I need to write a separate post for it soon). Then we went to the interior of this small island, to understand its volcanic past - all of the geology in Nisyros is volcanic in origin, making the surprisingly fertile island a unique place to visit.

The focus of our visit today was the phreatic (yes, that's a word, it describes volcanic activity based on steam) crater of Stefanos, set within the caldera, or collapsed crater, that is the interior of Nisyros. Stefanos is easily one of the strangest places we visit on any of our tours, a site that is fascinating for sure, and beautiful in its own way, but forbidding and otherworldy as well.

The story of Nisyros is a very long one. It is all based on the fact that the Aegean is a subduction zone, an area where the African continental plate is being pushed underneath the Eurasian one, a process that has been going on for millions of years, that takes place about 100km beneath the surface, and that makes the Aegean both tectonically and volcanically volatile. There are various active or dormant volcanoes in the area, among them Santorini, Milos, Methana and Kos.

Nisyros was first formed by magma welling up from the sea-bottom, cooling to form lava and creating a volcanic dome that eventually reached above sea level some 100,000 years ago. Over time, it became a classic conical volcano sticking out of the Aegean: an island. Two major eruptions, probably around 35,000 and 25,000 years ago, blew away the top of the island, forming the large inland caldera that now dominates its interior. Since then, there is evidence only for phreatic (steam-volcanic) activity.

What that means is that there is still an active magma chamber somewhere, but many kilometres beneath the island. Sea and rain water constantly seeps down towards it, and returns up to the surface in the form of superheated gases. Very occasionally, this leads to steam explosions on the surface, creating craters like Stefanos (the name means 'wreath', describing the circular shape of the steam crater). Stefanos, 5,000 years old, give or take, is a fine example of such a crater, a nearly circular opening in the pumice that fills the bottom of the Nisyros caldera. It is about 300m (1000ft) across and 25m (80f) deep, making it a major sight to behold. Stefanos is just one of a dozen or so steam craters in the Nisyros caldera, the most recent of which dates to the 1870s.

Walking in the volcanic caldera of Nisyros, Greece

Our guests are always fascinated visiting the Stefanos crater: we can walk through nearly all of it. Near its steep edges, there are a series of ever-changing fumaroles, emitting steam, often accompanied by a loud hissing noise, and usually smelling strongly of sulphur. Sulphurous oxides in bright greens and yellows are crystallising near the vents, giving the area a weird colour-scheme. In the middle of the crater, there is an also ever-changing area of holes, pools in the muddy flat terrain, sometimes dry and sometimes containing quantities of visibly boiling mud and water, bubbling loudly. Today was a bubbly day in the Stefanos crater...

Scientific understanding is one thing, but the Nisyros volcano has a much more poetic presence in Greek mythology. During the epic battle between the Twelve Gods and the Giants, the powers of chaos and disorder thrown up against the Gods by Gaia, the earth mother, Poseidon, the god of the sea, was pursuing the giant Polybotes (Polyvotis), racing across the Aegean Sea. In his anger, Poseidon rammed his trident into the island of Kos, pulled up a piece of it and threw it on top of Polybotes, trapping him underneath. Ever since, Polybotes is trying to escape, heaving and huffing, causing the steam, the foul odour and the frequent tremors that characterise Nisyros. Considering this violent tale, Nisyros is a surprisingly friendly, calm and welcoming island overall. The scene of Poseidon tossing Nisyros on top of Polybotes was depicted in one of the most prominent metopes of the Parthenon, above the entrance to its interior shrine!

We had a wonderful village dinner, including stuffed zucchini (courgette) flowers and slow-cooked local goat, in the village of Emborio, overlooking the caldera. To visit the amazing island of Nisyros join one of our expert-led gulet cruises in Greece.

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