It lies above a basement of Mesozoic limestone and a thinned crust of the Alpine orogen, with a mantle-crust transition at ~ 27 km depth. The volcanic edifice of Nisyros comprises a succession of calc-alkaline lavas and pyroclastic rocks, with a summit caldera of an average diameter of 4 km. Although the last magmatic volcanic activity on Nisyros dates back at least 18,000 years, the present geodynamic activity encompasses high seismic unrest and widespread fumarolic activity with surface temperatures of ~ 100°C and 350°C at a depth of 1’500m. Violent earthquakes and steam blasts accompanied the most recent hydrothermal eruptions in 1871–1873 and 1887 and left large hydrothermal explosion craters behind. Mud and hydrothermal vapor rich in CO2 and H2S were emitted from fracture zones which cut the caldera and extend towards the NNW through the vicinity of the village of Mandraki, into the island of Yali, and even towards Kos. In 1996 and 1997 seismic activity started with earthquakes of magnitudes up to 5.5 and with hypocenters down to 10 km depth around inferred magna reservoirs between 5 -12 km. The discharge of all hydrothermal craters in the caldera releases 68 tons of hydrothermal-volcanic derived CO2 and around 42 MW of thermal energy per day.

Nisyros Island has a long-standing colorful history since it is found at the crossroads between occidental and oriental cultures. Cycladic and Minoan relicts are spotted in the wild mountainous scenery of post-caldera domes, whereas witnesses from the Hellenistic epoch, Roman, Byzantine, Venetian and Ottoman times are represented in ancient caverns and spas, magnificent castles, churches and monasteries.