Overlapping craters of all sizes pockmark the Campi Flegrei volcanic region, which is nestled at the western edge of Naples, Italy, and stretches out into the Mediterranean Sea. Today, more than half a million people have settled near the slumbering volcano, whose rumblings and effusive gasses reflect the heat that still brews below. Twice in the last 60,000 years, large blasts of volcanic ash and rock have blanketed the region, and a smattering of smaller bursts have happened before and after each big eruption, including the most recent event in 1538.

For clues on what's happening in the magma chamber below, a team of researchers have now examined the chemistry of volcanic rocks and glass from historical eruptions, using