The Tyrrhenian Titan
Beneath the surface of the Tyrrhenian Sea, about 175 kilometers south of Naples, lies Marsili, the largest and highest active volcano in Europe. This enormous seamount is approximately 70 kilometers long and 30 kilometers wide, dwarfing terrestrial volcanoes like Mount Etna. It rises about 3,000 meters from the abyssal plain, its summit reaching to within 450 to 508 meters of the sea surface.
Marsili began forming between 0.7 and 1 million years ago in the Marsili Basin, a region of thin oceanic crust only about 10 kilometers thick. Its structure is a NNE-SSW elongated complex featuring multiple eruptive fractures and over 80 smaller adventitious cones. The volcano's activity is a result of the subduction of the Ionian tectonic plate. While its most recent eruptions are estimated to have occurred between 2,100 and 3,000 years ago, evidence of its active nature is abundant. Scientific monitoring has detected ongoing low-magnitude seismicity, hydrothermal venting, and significant heat flow from the volcano's crest. High heat flow measurements, reaching up to 500 mW/m2 in its central parts, combined with gravity and magnetic anomalies, point to the presence of a shallow magma chamber. Studies suggest the top of this magmatic reservoir sits approximately 2 to 3 kilometers below the volcano's crest.
A Hazard of Collapse
The primary hazard from Marsili is not a classic explosive eruption. The immense pressure of the overlying water, more than 500 meters deep, would likely suppress a major blast. The main threat comes from the structural instability of the volcano itself. A large section of Marsili's summit is composed of low-density, fractured rocks, significantly weakened by persistent hydrothermal activity. This ongoing alteration process makes the edifice fragile and prone to collapse.
A large-scale submarine landslide, potentially involving several cubic kilometers of rock, could displace a massive volume of water and generate a powerful tsunami. Numerical simulations of such a collapse show that tsunami waves could reach the Aeolian Islands in about 10 minutes and the coasts of Calabria and Sicily in just 20 minutes. The Italian National Institute of Geophysics and Volcanology (INGV) has identified Marsili as a dangerous volcano because of this collapse risk. Continuous monitoring using Ocean Bottom Seismometers (OBS) has recorded thousands of small seismic events and tremors, confirming the volcano's active state.