Italy's Campi Flegrei volcano hit by 150 earthquakes in just 5 hours

A wave of 150 earthquakes rocked the Campi Flegrei volcano near Naples last week — the biggest swarm in four decades.

The seismic swarm began May 20 just before 8 p.m. local time (2 p.m. ET), which included a magnitude 4.4 earthquake registered at a depth of 1.6 miles (2.5 kilometers) below the surface, experts wrote in a statement. The swarm lasted almost 5 hours, sparking panic among residents in nearby towns, but there were no reported injuries.

Thirty nine families were temporarily evacuated from their homes, and small tremors were still being felt around lunchtime May 21, Al Jazeera reported. 

"This is the most powerful seismic swarm in the last 40 years," Mauro Antonio Di Vito, a volcanologist and director of Italy's National Institute of Geophysics and Volcanology (INGV) Vesuvius Observatory, told the news channel.

Campi Flegrei is Europe's largest active caldera, and the volcano is showing signs it could be creeping toward an eruption. A caldera is a type of crater that forms when the roof of a volcano collapses into the magma chamber as it empties during an eruption. Campi Flegrei last erupted in 1538; in the months prior to that blast, the ground swelled and rose by 66 feet (20 meters), then subsided again as the volcano ejected its stores of magma.

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The ground beneath Pozzuoli, a town located near the center of the caldera, has been rising for the past 20 years, Christopher Kilburn, a professor of volcanology and geophysical hazards at University College London in the U.K., told Live Science in an interview in December 2023. Irregular patterns of seismicity started to pick up about 10 years ago and have been increasing ever since, Kilburn said.