The Italian name for the caldera - Campi Flegrei, or "burning fields"- is apt. The 12km-wide cauldron is the collapsed top of an ancient volcano, formed when the magma within finally blew. Though half of it is obscured beneath the crystal blue waters of the Mediterranean, the other half is studded with cinder cones and calderas from smaller eruptions. And the whole area seethes with hydrothermal activity: Sulfuric acid spews from active fumaroles; geysers spout water and steam and the ground froths with boiling mud; and earthquake swarms shudder through the region, 200km south of Rome.
And things seem to be heating up. Writing in the journal Nature on Tuesday, scientists report that the caldera is nearing a critical point at which decreased pressure on rising magma triggers a runaway release of gas and fluid, potentially leading to an eruption.
Forecasting volcanic eruptions is a famously dicey endeavour, and right now, it's impossible to say if and when Campi Flegrei might erupt, according to lead author Giovanni Chiodini, a volcanologist at the National Institute of Geophysics in Rome. But now more than ever, the caldera demands attention: An eruption would be devastating to the 500,000 people living in and around it.
The site's last major eruption happened over the course of a week in 1538, when it expelled enough new material to create the cinder cone mountain Monte Nuovo.
But the caldera itself is some 39,000 years old, formed by an eruption larger than anything else in the past 200,000 years of European history. A 2010 study in the journal Current Anthropology suggested that this prehistoric outburst - which spewed almost a trillion gallons of molten rock and released just as much sulphur into the atmosphere - set off a "volcanic winter" that led to the demise of the Neanderthals, who died out shortly afterward.
Today, the Campi Flegrei caldera is increasingly restless. For half a century, scientists have measured "bradyseism" events - slow movements of the ground - that are indicative of molten rock slowly filling the mountain's magma chamber. Significant uplift in the past decade prompted Italian authorities to raise the supervolcano's alert level from green (quiet) to yellow (scientific attention) in 2012.
"These areas can give rise to the only eruptions that can have global catastrophic effects comparable to major meteorite impacts," Giuseppe De Natale, head of a drilling project to monitor the caldera, told Reuters after that change was made in 2012.
Now, Chiodini and his colleagues have identified the volcano's "critical degassing pressure" - a vital data point in understanding the likelihood of an eruption. As molten rock from the Earth's interior rises through the crust, it is subject to less pressure, and this decline in pressure causes volatile gases dissolved within it to be released. At the critical degassing pressure point, this process accelerates tenfold. Huge amounts of steam are injected into the surrounding rock.
If the magma loses too much water, it may harden and cease its upward motion, stopping the eruption in its tracks.
Alternatively, the injections of steam could destabilise the rock, accelerate the deformation process and ultimately cause the volcano to blow.
Chiodini said scientists have seen an increase in ground deformation and low-level seismic activity around the caldera in recent years. This pattern compares with activity seen around similar volcanoes before their eruptions.
This doesn't mean residents of Naples should be heading for cover.
"In general, unfortunately, volcanology is not a precise science," Chiodini wrote in an email. "We have many uncertainties and long-term previsions are at the moment not possible! For example, the process that we describe could evolve in both directions: toward pre-eruptive conditions or to the finish of the volcanic unrest."
What's more, volcanoes operate on time scales far beyond the scope of most scientific records. Campi Flegrei has had a major eruption only twice in the past 40,000 years, and both happened long before the invention of the first writing systems, let alone seismometers. There are written accounts of the 1538 eruption, but there are limits to the scientific insight those descriptions they provide.
What they lack in data, however, they make up for in vividness. Take this recollection, written by Italian philosopher Simone Porzio:
"The large tract of land which lay between the foot of the mountain . . . and the sea . . . was seen to rise and take the form of the newly produced mountain. And on the same day, at the second hour of the night, this mount of earth opened like a mouth, with a great roaring, vomiting much fire and pumice and stones."