Lurking beneath the Mediterranean, between Sicily and the Italian mainland, lies a submarine volcano, which is showing telltale signs of being unstable.

News reports this week declared that its flanks could cave in, triggering a tsunami that would swamp southern Italy. But geologists caution that while the threat is real, it is impossible to say when the volcano might collapse.
Enzo Boschi of the Institute for Geophysics and Volcanology in Rome, took a peek at the internal structure of the Marsili seamount using remote sensing methods, which included measuring how the volcano affects Earth’s gravitational field.
Marsili has a hidden but active magma chamber and lies beneath the surface of the Tyrrhenian Sea, which is part of the Mediterranean west of Italy.
The team found that parts of the seamount’s steep flanks were made of porous volcanic material, leaving them weak and unstable.
They warn that an earthquake nearby or an eruption within Marsili itself would be enough to cause the seamount’s flank to collapse, triggering a tsunami that would strike the coasts of southern Italy, Sicily, and Sardinia.
It wouldn’t be the first time the seamount has collapsed. The distribution of material around Marsili hints that smaller collapses have occurred there before.

In addition, the Vavilov seamount, around 200 kilometers north-west of Marsili, is made of similarly porous material. It seems to have lost an entire 40-kilometer-long flank in one catastrophic event, say the researchers.

Bill McGuire of University College London cautions that it’s impossible to say when the volcano might erupt in the future.