The caldera of Nisyros, the central crater of its volcano, is a landscape of special geological and natural interest, which has been included and protected by the program of the European Union "Natura 2000".
The volcano of Nisyros is the newest active volcano in Greece, its oldest rocks do not exceed 150,000 years old. Up to 60,000 years ago today, hundreds of small explosions built a cone that rose about 800 meters above the sea and was about the perimeter of present-day Nisyros.
Two huge eruptions, 55,000 and 45,000 years ago today, destroy the central part of the volcano, which collapses and creates the large central crater, the Nisyros caldera, with a diameter of 4 km, the bottom of which you visit today. Between 30,000 and 15,000 large lava hills are created that fill the western half of the caldera, with the highest that of Pr. Elias (698 m. Height).
Since then the volcano remains calm.
However, the incandescent rock, located a few kilometers below the caldera and its hot gases, heats the sea and rain water that circulates in the shallowest rocks and turns it into a superheated fluid with temperatures approaching 500 degrees Celsius very deep. This fuels intense hydrothermal activity, creates the escape of hot gases, fumes, deposits of sulfur minerals and alters the rocks around the site of this action. The hot gases consist mainly of water vapor, but they also contain a sufficient amount of carbon dioxide as well as hydrogen sulfide, which is due to the suffocating characteristic odor that prevails in the space.
At intervals, usually during major seismic crises, the superheated fluid shakes in the air the rocks that prevent it from reaching the surface causing hydrothermal explosions and creating hydrothermal craters. Ten such craters are kept at the bottom of the caldera, most in very good condition. Stefanos, the crater you visit, is the largest and most spectacular of them. With a maximum diameter of 330 meters it is one of the most spectacular and largest hydrothermal craters in the world. The hydrothermal explosion that caused it was huge and took place about 6000 years ago. A few hundred meters west of Stefanos, in the area of Lofos, there are also six spectacular craters. Two of them, Fleghethros and Polyvotis, were created in 1873. Mikros Polyvotis is the product of the last hydrothermal explosion of Nisyros, which took place in 1887.