Moving one and a half inches a year sounds pretty slow, but it’s lightning quick for a mountain. It’s also exactly how quickly Sicily’s great Mount Etna is sliding into the Ionian Sea. At over 10,000 feet in elevation, Mount Etna is Italy’s largest active volcano, but it’s slowly being humbled as it sinks deeper into the water around it. Now, in a paper published Wednesday in Science Advances, scientists explain why the massive monument is sliding into the sea — and warn locals of a potentially disastrous collapse.

The destruction of Mount Etna would be catastrophic, but it wouldn’t be unprecedented. “We know from the geological record that volcanoes with gravitational instability have collapsed,” Morelia Urlaub, Ph.D., a marine geodynamics researcher at GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany and the paper’s first author, tells Inverse. “So there could be a chance that Etna’s flank may also collapse and cause a landslide that rapidly enters into the sea — which would cause a tsunami.” Understanding how and why the volcano moves and shifts will help scientists inform the public about the risks they face by living in the shadow of Mount Etna.

The volcano’s precarious state wouldn’t be a big deal if it was in the middle of nowhere. Unfortunately, it’s surrounded by cities, towns, and farms, where the local wine industry benefits from the fertile volcanic soil. About 8,000 years ago, its eastern flank is thought to have collapsed, triggering a tsunami that destroyed a coastal community in present-day Israel, over 1,000 miles away across the Mediterranean Sea.

Research Vessel Poseidon deploys transponders across the seafloor at Mount Etna's flank to detect changes in the seafloor geology. Dr. Felix Gross

Previous explanations for the volcano’s descent centered on the movement of magma or the simple force of gravity, but it’s been impossible to separate the effects of these two forces. To get to the bottom of the situation, Urlaub and her team devised an ingenious strategy that involved mapping the seafloor with electronic devices.

They placed an array of five transponders on the bottom of the ocean, with some on each side of the boundary between the volcano’s flank and the rest of the seafloor. These transponders, which were about 25 miles from the volcano’s central crater, transmitted their positions to researchers, painting a continuous picture of the volcano’s flank’s movement. They remained there from April 2016 to July 2017, and for most of this time, they remained at the same positions. But over 10 days in May 2017, they recorded a shift that changed their positions about 1.5 inches (4 centimeters) relative to one another. This shift corresponded with a shift in the fault line that didn’t cause an earthquake, indicating that Mount Etna shifts even more frequently than researchers had ever suspected.

“The study shows that the flank movement extends well into the sea and affects a much larger area than previously known,” says Urlaub. “The fact that there is movement so far away from the actual heart of the volcano (the magma chamber) means that flank movement is not caused by rising magma that gives the southeastern flank a horizontal push (as previously thought), but that the main driver of the sliding flank is gravity.”

From Riposto Harbor, you can see the unstable southeastern flank of Mount Etna. Dr. Alessandro Bonforte

In short, while previous research suggested that a huge volcanic eruption might be the event that destabilizes Mount Etna and causes a large portion of it to collapse, it turns out that the more common occurrence of tectonic plate boundary shifts could actually be the factor that pushes the volcano over the edge. In fact, such a shift and collapse could actually trigger an eruption, multiplying the potential damage. Taken altogether, these findings suggest that in Mount Etna’s history, collapses typically happened not as a result of flashy volcanic eruptions but as the result of much less impressive plate shifts.

So how much of a danger does Mount Etna currently pose to the fisherman, wine growers, and other residents of Catania, the city that lies on the volcano’s collapsing flank?

“Currently, it is hardly noticeable for people who live there,” says Urlaub. “Some roads need to be re-paved frequently due to fractures, and houses need new paint to cover fractures.” So for now, it sounds like Sicilians who live in the shadow of Mount Etna are safe, but further research will reveal how safe their children and grandchildren will be.