The ground on the western edges of Naples, Italy is rising, spurring worries of a possible volcanic eruption, but scientists now think they know exactly what is causing the uplift and may be able to better predict any potential eruption.

Using GPS measurements, a group of scientists at the National Institute of Geophysics and Vulcanology in Italy monitored the ground’s motions for several years, and based on the patterns they observed, they believe the uplifting is caused by magma intruding from a shallow chamber.

The rising motions of the ground reached a peak rate of about three feet per year during two major uplift episodes in the last few decades. Some previous episode of the alternate uplifting and subsidence left its mark: Bore holes from mollusks can be found on Roman pillars in the area, indicating the ground once subsided below water and has since risen up again.

The land on the western edge of Naples, called the Campi Flegrei, is a large, ancient volcanic caldera about 6 kilometers (about 4 miles) across. A caldera forms when a volcano collapses into itself after the underlying magma chamber empties, usually from an eruption.

Campi Flegrei last erupted in 1538. It is part of a larger volcanic arc that includes Mount Vesuvius.

Though the caldera has many extinct craters, at least one, called the Solfatara for the jets of sulfur fumes it emits, is still active. Because of the high population density in the caldera, it is one of the most dangerous volcanic areas in the world.

The Campi Flegrei is known for a phenomenon called bradyseism, or the alternate uplifting and subsiding of the ground over the span of several years. This activity centers on the town of Pozzuoli, which suffered damage from two major uplifts between 1970 and 1972 and between 1982 and 1984.

The scientists have not said whether an eruption might occur anytime soon. But they say that continued GPS observations will allow them to monitor the movement of magma at this and other volcanoes and  help improve predictions of eruptions worldwide.

The research, announced yesterday, is detailed in the journal Geophysical Research Letters.