Nisyros' Volcano

Nisyros is a round volcanic island in the southern Dodecanese, south of Kos, and its middle is missing. A caldera three to four kilometres across takes up the centre of the island, with the villages set on the rim and along the coast around it. Nisyros is the youngest of the major volcanic centres in Greece and the volcano is classed as active, although it has produced no lava in recorded history.

How the island was built

Nisyros was raised by volcanic activity over roughly the past 150,000 years, in three broad eruptive stages that laid down andesite, then dacite, then rhyolite. The last of them ended in the collapse that formed the caldera. The sequence is legible in the road cuts and sea cliffs: layers of lava, pumice and ash in grey, red and yellow stacked on top of one another, with rhyolitic domes standing along the western rim.

The southeastern part of the caldera floor is a flat plain called Lakki, and this is where the hydrothermal craters sit. Stefanos is the largest and best preserved of them, an oval pit between 260 and 330 metres across and about 27 metres deep, with a floor of cracked grey clay, fumaroles at the edges and sulphur crystals growing around the vents. Nearby are the smaller craters of Megalos and Mikros Polyvotis, and Alexandros, also called Flegethron. All of them were opened by steam explosions rather than by lava.

Poseidon and the giant Polybotes

The Greek account of how the island came to be belongs to the Gigantomachy, the war between the Olympian gods and the Giants, not the earlier war against the Titans with which it is often confused. Poseidon pursued the giant Polybotes across the Aegean as far as Kos, tore off a piece of that island and hurled it at him, pinning him underneath. The rock became Nisyros, and the steam and rumbling were explained as the giant struggling below. The story survives on the map: the two Polyvotis craters on the Lakki plain carry his name. Stefanos, the crater most visitors walk into, does not come from mythology at all, despite what some guides suggest.

Eruptions and monitoring

Four eruptions are recorded in historical times, all of them small and all of them steam or ash rather than lava. Ash eruptions in 1871 and 1873 and a steam blast in 1887 to 1888 opened several of the craters now on the Lakki plain, and there has been no eruption since. What continues is the system underneath. Nisyros holds one of the highest-temperature geothermal fields in Greece, with fluids above 350 degrees Celsius at depth, and a seismic crisis in the mid-1990s came with measurable uplift of the ground. The volcano is monitored continuously, and the craters are open to visitors on that basis.

The volcano museum at Nikia

Nikia sits on the caldera rim on the southeastern side and looks straight down onto Lakki and Stefanos. The Volcanological Museum, the first of its kind in Greece, opened in 2008 on the village square and sets out how the island was assembled: eruption types, the rocks underfoot, the monitoring equipment and the wider volcanic arc that runs from Methana through Milos and Santorini to Nisyros. From the terrace by the chapel above the village the whole crater field is visible at once. A path drops from Nikia to the caldera floor, about two kilometres and forty-five minutes down, for anyone who would rather walk in than drive to the car park.

Getting to the island

Ferries and excursion boats run to Mandraki from Kos, but the day-trip schedule leaves only a couple of hours ashore and lands everyone on the crater floor at the same time. The caldera is at its best outside that window, which is why the island is best reached during a gulet cruise along the Greece coast, arriving on your own timetable rather than the excursion boat's. Mandraki has a harbour on the northwest coast, and the road up to the caldera starts there.

Walking into Stefanos

The descent into Stefanos is on a marked path from the caldera floor. Inside, the ground is warm, the crust around the vents is thin and will give way under a boot, and sulphur fumes collect in the hollows on still days, so the marked route is not a formality. Sturdy shoes, water and sun cover are the practical requirements: there is no shade anywhere on the plain and the pale clay floor throws the heat back. Early morning and late afternoon are cooler as well as quieter.

The Nisyros Geopark maintains the paths and viewpoints around the caldera and runs an app for the Lakki field, with maps, photographs and 360-degree panoramas. It is most useful once you are down among the craters and trying to work out which vent is which.