“The danger isn’t the eruption, but the possible underwater landslides,” says Ventura. If the seismic movements beforehand, or the explosion itself, led one of its flanks to collapse, it would displace such a large volume of water that it would trigger a tsunami.

The devastation of Naples

We now know that submarine landslides from the Aeolian arc of volcanoes could have contributed to past tragedies.

In 1343, for instance, the poet Petrarch described a terrible sea storm that devastated the Bay of Naples, leading to hundreds of deaths.

A recent study by Sara Levi at the State University of New York and colleagues suggests this may have been the result of a tsunami, originating on Stromboli. Analysing archaeological evidence from the volcano, the team found evidence of an ancient landslip,  resulting in a tsunami that reached the Calabrian coast.

A more recent eruption at Stromboli, in 2002, led to two tsunamis. The waves only affected the island itself, however, without reaching the mainland – and no one was killed by the surge.

Unfortunately, we can’t yet calculate the precise threat from Marsili. “We just don’t have enough data,” says Glauco Gallotti, a physicist at the University of Bologna. But there are good reasons to think it may pose a danger, he says. For one thing, the continuing hydrothermal activity could have weakened the volcano’s rocks. What’s more, recordings of microearthquakes shows that lava is still churning in the magma chamber, he says. For both these reasons, we should take the possibility seriously with further research.

In a paper published earlier this year, Gallotti’s team considered five different scenarios, each examining the effects of different possible landslides. In the first couple of cases, the displacement of water was minimal – leading to a wave of just a few centimetres in height.

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A collapse on the north-western flank, however, proved to be more serious – leading to a 3-4m high (10-13ft) wave reaching southern Campania, and a 2-3m high (6.6-10ft) wave hitting Calabria and Sicily within 30 minutes of the event. This is worth taking seriously, since a “scar” on Marsili’s seamount suggests that a similar landslide may have occurred in the past. “And it could have generated waves between 1m and 3m high [3-10ft],” says Gallotti.

The worst-case scenario involved the collapse of the southern-central summit and the eastern flank. According to Gallotti’s calculations, it would result in a 20m-high (66ft) wave reaching Sicily and Calabria within 20 minutes. Further research will be needed to assess the probability that this will occur, he emphasises. “But we can’t [yet] rule out the possibility.

If an eruption does happen, the human cost may depend on the time of year, and whether or not the tsunami arrived in the peak of the tourist season. “The south of Italy is vastly populated in the summer,” Gallotti says. The elevation of Italy’s coastline means that people should be safe if they were around one kilometre inland, he estimates.

Seismic sisters

Ventura agrees that these risks should be explored. “It’s clear that Marsili needs to be monitored for possible instabilities on its flanks.”

As the events at Stromboli had shown, Marsili may be the biggest volcano in Europe, but its sisters could also pose significant threats. “It’s worth remembering that in the Tyrrhenian Sea, as in the Strait of Sicily, there are at least 70 underwater volcanoes, whose story, in some cases, is totally unknown,” says Ventura.

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Of particular concern is the Palinuro volcano, located 65km (40 miles) off the coast of Cilento. According to Gallotti, recent studies show that the volcanic complex has a 150m-layer (490ft) of loose material that could be displaced with seismic movements. Ideally, future surveys will provide further details of that structure and its chances of collapse.

If the risk is significant, the Italian government may need to take action to pre-empt the potential disaster. At the moment, there is no good monitoring system to warn Italian people of an impending tsunami. But it need not be too difficult to build, Gallotti says. He points out that a network of buoys, fitted with motion sensors, should be able to detect characteristic differences in the sea’s movements that signal the emergence of a tsunami.

This could send an automatic SMS alert to people in the areas affected – allowing them to reach higher ground before the tsunami hit.

Ventura suggests it may also be possible to monitor the movements of the volcano itself. “In the last 40 years the technology for monitoring active volcanoes has made huge strides,” he says.

In addition to an early warning system, Gallotti would also like to see greater awareness of the potential danger – among the public and policy makers. As evidence, he cites a paper by Teresita Gravina, at Guglielmo Marconi University, Nicola Mari at the University of Glasgow, and colleagues, which recently assessed people’s risk perception in southern Italy.

In general, people were aware that tsunamis could occur in the area, but their knowledge was hazy. When asked about the most recent events, for example, only 3.3 percent of the people who answered mentioned the 2002 tsunami at Stromboli, for instance. Most believed themselves to be ill-informed about the events, with limited knowledge of the tsunami’s potential causes or the best ways to act, were another to occur.

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This is quite serious,” Gravina and Mari told BBC Future in an email. “It’s probably due to the fact that in Italy there’s little communication on these subjects. To know how a seismic phenomenon, or a tsunami, forms is very important, because you can assess completely different scenarios and the different behaviours that would be necessary to avoid the danger.” Any attempts to design an effective early warning system would need to consider the behaviours of the different communities, they say.

(We’ve already seen this with Covid-19 – two populations can react to exactly same information in very different ways.)

There have been some positive steps. A recent project in Salerno, organised by Italy’s Civil Protection Department, attempted to simulate the emergency caused by a tsunami. “This could have increased their awareness of the risk,” Gravina and Mari say. But for most people, the danger of an underwater landslide, caused by a submarine volcano, is still little understood – meaning that much more work is needed to educate people of the possibility.

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