Underneath the vineyards and thermal spas in southern Italy, magma churns to create one of the world’s most active volcanic systems in a region known as Campi Flegrei.

Outside of Naples, Campi Flegrei does not appear as a typical volcanic mountain but is a bowl-shaped depression peppered with craters. Smelly steam oozes from vents, mud gurgles from pools, and small earthquakes send shocks to hundreds of thousands of residents living in the volcano’s mouth. Mythology suggests Campi Flegrei, meaning “burning fields” in Italian, is associated with the gates to hell.

It’s also dubbed a “supervolcano” — a rare but unofficial label given to those that have produced the most intense eruptions in Earth’s history. Campi Flegrei’s super outburst occurred around 39,000 years ago (determined through rock records) and spewed gases and nearly a trillion gallons of molten rock, blocking sunlight and triggering intense cooling. The most recent eruption, much smaller, occurred in 1538 and created a roughly 120-meter-tall mound.

Now, recent months of earthquake activity at Campi Flegrei — more than 2,500 earthquakes as intense as a 4.3-magnitude since September — has stirred concerns that the volcano could super-strike again soon. But researchers say that’s not how supervolcanoes work and cast doubts for a prophetic outburst.

What does it mean to be a supervolcano?

“When a volcano is called a supervolcano, what we really mean is it had a super eruption once, at least, in the past,” said Christopher Kilburn, a volcanologist at University College London. “But that doesn’t mean that it’s going to have other super eruptions in the future. . . . Very, very, very large eruptions are much, much rarer.”

Scientists can’t see what is stirring below the surface of Campi Flegrei with their naked eyes, but Kilburn said the recent activity could be underground molten rock and fluids readjusting themselves. Those movements appear as earthquakes on the surface.

“This, by itself, doesn’t mean there’s going to be an eruption,” Kilburn said. The volcano has shown land deformations and earthquakes in the past, but eruptions didn’t follow. But because the activity is stirring after a long time, “it’s natural just to be a little bit concerned that this might be happening.”

Out of more than 1,000 known volcanoes in the world, only about 20 are supposedly supervolcanoes. Technically, they are defined as those that register the highest on the volcanic explosivity index, which runs from V0 (nonexplosive) to V8 (colossal eruptions). Such a super eruption ejects a volume of around 1,000-cubic kilometers or more — about a thousand times bigger than Mount St. Helens (V5), which caused mudslides, fires, floods and more than 50 deaths in 1980.

The last V8 eruption occurred around 27,000 years ago in Taupo, New Zealand.

Such intense volcanic outbursts usually leave behind a depression known as a caldera, instead of a volcanic cone. Kilburn said that’s because the eruptions are throwing out a large volume of material — molten rock stored a few miles below the surface — in a very short amount of time. The ground becomes unstable and sinks.

Unless you’re looking out for these depressions, he said you may miss it.

“You can drive across the caldera and come out the other side and not really appreciate the fact that’s what you’ve done because the changes are quite gentle,” Kilburn said.

Once the massive eruption ends, Kilburn said the volcano resets and becomes “ordinary,” sometimes producing more normal-size eruptions across the caldera floor. In other words, there’s nothing inherently “super” about a supervolcano after it erupts, making the label a bit misleading.

Yellowstone, one of the world’s most famous supervolcanoes, measures 30 by 45 miles and welcomes millions of tourists to its park. Its largest eruption occurred 2.1 million years ago, ejecting more than 2,400 cubic kilometers of material. Like many caldera systems, the majority of Yellowstone’s eruptions have since been much smaller.

‘A made-up word’

Supervolcano is “a made-up word,” said volcanologist Michael Poland, scientist in charge at the Yellowstone Volcano Observatory. “I think it’s misleading. I think it’s misapplied. I can’t stand that term. I wish it would go into the dustbin, but it’s too sexy.”

Like superman or superstar, supervolcano sounds too Hollywood for his taste. It implies an apocalyptic-like explosion, but no explosive volcanic eruption has caused a mass extinction to our knowledge, he said.

The largest volcanic explosion in the geologic record is thought to have occurred in Toba, Indonesia, around 74,000 years ago, registering a V8 on the volcanic explosivity index. Some scientists initially speculated that the eruption almost wiped out humanity because populations declined shortly after, but archaeological evidence showed Homo sapiens farther away were thriving after the eruption.

“No explosive volcanic eruption that we know of has ever been associated with a mass extinction of plant or animal life,” said Poland, who’s also a scientist at the U.S. Geological Survey (USGS). “That’s not to say it wouldn’t be devastating or hard to live through.”

Many speculate on what would happen if Yellowstone had another super eruption. The USGS said surrounding states would receive fast, hot avalanches of volcanic ash, pumice, gases and rocks. Ashfall could remain thick hundreds of miles away and be transported across the world. Small aerosol particles emitted from the volcano would reflect the sunlight back into space, causing cooling on the surface and affect agriculture.

But scientists are skeptical that a super eruption could happen again at Yellowstone. The volcano may not even have enough molten magma underneath its caldera to instigate an eruption, according to USGS.

Additionally, Kilburn doesn’t think “anybody’s thinking there’s going to be another super eruption” at Campi Flegrei in the near future. But a smaller eruption could have major effects given more than 1 million people live in and around the area. Local officials send out alerts based on the volcano’s activity and prepare evacuation plans.

Local authorities have “to take the possibility of an eruption into account,” Kilburn said. “Not saying that it’s likely, but it would be remiss if they ignored that option.”