of a volcanic eruption in the coming days”.

These ominous signs do not, though, guarantee that an eruption — defined as magma breaking through the Earth’s crust — will follow. Evgenia Ilyinskaya, a Leeds university volcanologist who grew up in Iceland and has helped to catalogue the country’s volcanoes, told me on Monday that seismic activity is waning, “but this doesn’t indicate either way whether an eruption is more or less likely. In the most recent eruptions in the same part of Iceland, we had earthquakes diminishing or stopping altogether a few days before eruptions broke out.”

To gauge what is happening, seismic information is combined with satellite imagery of how the ground is rising or falling, plus ground-based measurements that can detect how close to the surface the magma has risen. Even when deformation and depth are factored in, however, there is no formula governing when and where — or even if — a volcano will blow its top.

The order for Grindavik’s evacuation, Ilyinskaya says, instead resulted from a carefully weighed risk analysis incorporating several fast-changing factors: the high speed of ground deformation; the large volume of magma coming up; and proximity to the town. The underground magma channel now stretches to about 15km long, some of it under the Atlantic; in some places the magma is pooling less than 1km below the surface.

The current most likely scenario is a small eruption close to the town. The actual point of eruption matters: a “submarine” eruption through the sea floor is regarded as more disruptive than a land-based “fountaining” event because of the potential for an ash cloud, as well as noxious gases such as sulphur dioxide as seawater boils. Even so, any such incident should create less havoc than the 2010 eruption, which closed much of European airspace for six days.

As time passes, especially if no eruption materialises, the question of whether residents can return home will become more pressing. “It’s hugely difficult because people’s lives are on the line if you get it wrong,” Ilyinskaya says, citing the 2019 eruption of White Island (also known as Whakaari) in New Zealand, which killed 22 tourists and led to convictions over lax safety procedures. A culture of trust between Icelanders, scientists and decision makers, she points out, makes things easier.

A similar dilemma faces residents of Campi Flegrei, or Phlegraean Fields, the vast crater of an Italian supervolcano near Mount Vesuvius, which destroyed Pompeii in AD79. The area experienced a 4.2 magnitude earthquake last month, the biggest for 40 years. The civil protection agency and government met this month to thrash out evacuation drills for an estimated 500,000 residents in the “red zone”.

That so many choose to live amid these natural threats might seem odd, but the hard-to-quantify risks come with ample rewards. Campi Flegrei has drawn Roman emperors and commoners alike to its mild climate, rich soils and hot springs; Iceland’s Blue Lagoon, with its spalike waters, is similarly alluring, with the peninsula also home to a geothermal energy plant.

Still, when tectonic push comes to continental shove, nature remains in unpredictable command.