Nisyros Island

Nisyros is a roughly circular volcanic island in the Dodecanese, set between Kos and Tilos in the southeastern Aegean. The main island spans about 41.6 square kilometres with a diameter close to 8 kilometres, and the 2021 census recorded 1,048 residents across the municipality, most concentrated in the port town of Mandraki, home to around 660 people, with the smaller villages of Paloi, Nikia and Emporeios making up the rest.

A Volcano That Has Never Stopped Breathing

The island owes its shape to a wide volcanic caldera, 3 to 4 kilometres across, formed by eruptions within roughly the last 150,000 years across three separate eruptive stages. The rim climbs to 698 metres at its highest point, while the flat floor inside it, known locally as Lakki, is pitted with hydrothermal craters. The largest of them, Stefanos, measures roughly 260 by 350 metres and drops around 27 metres below the surrounding plain, making it one of the biggest and most accessible craters of its kind in the world. A short path leads down onto its floor, where mud pools bubble and fumaroles vent steam at temperatures above 100 degrees Celsius, crusting the ground in yellow sulphur.

Stefanos is far from alone. More than eight older explosion craters, some up to 300 metres wide, scar the caldera floor, evidence that steam-driven blasts have recurred repeatedly over the past few thousand years. Nisyros has four eruptions on the historical record, all modest events rated VEI 2: small ash eruptions in 1871 and 1873, and a final steam explosion in 1888. Nothing violent has occurred since, and the volcano is classified as active but not erupting.

Geothermal Heat and the Island's Hot Springs

Beneath the surface lies one of the most important high-enthalpy geothermal fields in Greece, with fluids exceeding 350 degrees Celsius at pressures of around 18 bar and an estimated potential above 50 megawatts. Plans by the national power corporation to exploit it collapsed after a local referendum in 1997, in which 87 percent of islanders voted against development, and the field has stayed untapped since.

The same heat surfaces harmlessly along the shoreline. The best known baths sit at Loutra, about 1,500 metres north of Mandraki, where a municipal thermal spring has been in use since 1872 and reaches roughly 46 degrees Celsius. The bath buildings went up in stages between 1885 and 1912, when Nisyros drew cure-seekers from across the Dodecanese, the coast of Asia Minor and even Egypt. Thermiani at Paloi and the cove of Avlaki below Nikia offer the same water in rougher form, the sea at Avlaki warmed to close to 59 degrees where the spring emerges.

Myth, Ancient Walls and a Crusader Castle

Greek myth explains the island's shape neatly enough: Poseidon is said to have torn a piece off Kos and hurled it at the giant Polybotes, pinning him beneath what became Nisyros. The island was known in antiquity as Porphyris and is named in Homer's Iliad among the contingents sailing for Troy. Above Mandraki stand the massive walls of Palaiokastro, the ancient acropolis, dated to the 5th century BC and ranked among the best-preserved ancient fortifications in the Aegean.

Later rulers left their marks on the same cliff. The Knights Hospitaller settled here in 1315 and built a castle above the harbour; within its walls sits the monastery of Panagia Spiliani, the Virgin of the Cave, reached by a steep stepped lane climbing out of Mandraki. The Ottomans took the island in 1566, Italy occupied it in 1911 during the Italo-Turkish War, and Nisyros was finally united with Greece in 1947 along with the rest of the Dodecanese.

Villages, Black Sand and a UNESCO Geopark

The hillside villages of Nikia and Emporeios offer sweeping views down into the volcanic basin, and marked footpaths link them to the crater floor for walkers who would rather not take the road. The coastline is mostly rock and shingle, but the eastern side hides dark volcanic sand: Pachia Ammos runs for about half a kilometre behind low dunes of coarse black grit, while the string of coves at Lies gives the same volcanic colouring in smaller, quieter pockets.

Offshore, the islet of Gyali is composed almost entirely of pumice and perlite, with obsidian deposits as well, and quarrying there has long underwritten the municipality's finances. The whole volcanic complex gained international standing when Nisyros was named a UNESCO Global Geopark in 2026, following approval by the Global Geoparks Council in September 2025. The geopark covers roughly 481 square kilometres, taking in the islets of Pachia, Strongyli, Pergousa and Kandeliousa and the sea between them, and protects 24 geosites alongside two Natura 2000 areas, some 450 plant species and 85 recorded bird species.

Climate, Ferries and When to Visit

Nisyros has a hot semi-arid climate, averaging around 20 degrees Celsius across the year with only about 350 millimetres of rainfall, so a summer visit means heat and almost no shade inside the crater, which makes early morning the sensible time to walk it. Etesian winds sweep the island from June through August, bending trees along its exposed eastern and western flanks and taking the edge off the temperature down at the coast.

Nisyros connects by ferry to Kos, Rhodes and Piraeus, with daily summer boats from Kardamena on Kos, and its place along these routes makes it a classic stop on a Greece blue cruise itinerary, letting sailors pair a walk around the caldera with an afternoon anchored off Mandraki's harbour. Before leaving, it is worth seeking out soumada, the island's almond cordial, sold in bottles along the waterfront.