In the Bay of Naples, Europe's most notorious giant is showing signs of reawakening from its long slumber.

Campi Flegrei, a name that aptly translates as "burning fields", is a supervolcano. It consists of a vast and complex network of underground chambers that formed hundreds of thousands of years ago, stretching from the outskirts of Naples to underneath the Mediterranean Sea. About half a million people live in Campi Flegrei's seven-mile-long caldera, which was formed by vast eruptions 200,000, 39,000, 35,000 and 12,000 years ago.

The past 500 years have been fairly peaceful ones for Campi Flegrei. There have been no eruptions at all since 1538, and that was a comparatively small event that resulted in the formation of the "New Mountain", Monte Nuovo. But recent events suggest that this period of quiescence may be coming to an end.

An acceleration of processes causing deformation and heating within the caldera saw the Italian government raise the volcano's threat level in December 2016. Fears are growing that magma deep inside Campi Flegrei could be reaching the "critical degassing pressure", where a sudden large-scale release of volcanic gases could abruptly inject heat into surrounding hydrothermal fluids and rocks. When this happens on a significant scale, it can cause catastrophic rock failure within the volcano, triggering an eruption. In line with this, a study published in May 2017 found evidence that the supervolcano has been building towards an eruption for decades.

But the difficult question is not if, but when, and just how big an event this would be.