A piercing alarm burst from millions of cellphones, a signal to hundreds of thousands of people to pack their bags and flee one of Europe’s most dangerous volcanoes. But most of the Italians who heard it shrugged.

It was around 5 o’clock on a Friday afternoon, and the alert wasn’t announcing a real crisis.

Instead, it was part of a four-day drill this month, coordinated by the Italian civil protection department, to prepare a densely populated area near Naples for the day its residents might face a host of volcanic perils: The ground buckling underfoot. Ribbons of toxic fumes. Exploding boils of molten rock.

The threat does not loom on the horizon, like nearby Mount Vesuvius to the east. Instead, an eight-mile-wide caldera — riddled with volcanoes — is recessed in the earth and sea west of Naples, forming what is called the Campi Flegrei, or “burning fields.”

Most experts believe an eruption remains a remote possibility, but volcanic activity — hundreds of mostly minor earthquakes, along with a measurable rise and subsidence of the earth — has picked up considerably in recent years, panicking some residents and putting the authorities on high alert.