Now scientists say a SECOND super-volcano is poised to blow: Warning over Italy's Campi Flegrei comes after experts said Long Valley Caldera in California could erupt

The Italian government is planning for a possible evacuation of more than 300,000 people due to a nearby dormant super volcano showing signs of an eruption - one month after American researchers found one rumbling in the US. 

Campo Flegrei, west of Naples, has experienced more than 2,500 earthquakes in the past three months, with the largest hitting a 4.0 magnitude, which is a similar scenario that led to its 1583 eruption.

Scientists have predicted that if this sleeping giant blows, it could create a plum large enough to plunge Earth into a global winter for years.

The warning comes months after California researchers detected more than 2,000 earthquakes rumbling through the state's Long Valley Caldera that they said 'are precursors of an eruption.' 

The term 'supervolcano' implies a volcanic center with an eruption of magnitude 8 on the (VEI), meaning that it erupted at least 240 cubic miles of material at one point.

Explosive events of this size erupt so much magma that a circular-shaped collapse feature called a caldera forms above the evacuated magma storage region.

If Campi Flegrei were to blow, experts believe it would unleash molten lava and volcanic gases into the stratosphere, Live Science reports.

Tsunamis more than 100 feet high would form and spread the plum of toxic ash that could plunge Earth into a global winter for years. 

Italy's supervolcano has been making headlines as officials mull over evacuating those living there due to the earthquakes that have reached a 4.2 magnitude and sightings of sulfurous fumes escaping from the surface.

And it is not just the more than 360,000 people who live at the mouth of the volcano who are at risk - there are about 2.3 million living on or around it. 

Scientists have claimed that the Campi Flegrei crater was formed 39,000 years ago in a blast, the largest in Europe in the past 200,000 years.

Campi Flegrei last erupted in 1538, though on a much smaller scale. 

The devastating Mount Vesuvius in 79 released a cloud of super-heated material and gases 21 miles into the sky,  ejecting molten rock, pulverized pumice and hot ash at 1.5 million tons per second.

But Campi Flegrei is much larger than Mount Vesuvius.

Approximately 10 earthquakes were detected around Campi Flegrei on Wednesday, but none exceeded a 1.1 magnitude. 

The last large tremor hit on October 2 - a 4.0 magnitude.

'A few large regional earthquakes (greater than magnitude 6) are related to a subsequent eruption or to some type of unrest at a nearby volcano,' according to the United States Geological Survey.

'However, volcanoes can only be triggered into eruption by nearby tectonic earthquakes if they are already poised to erupt.'

The big quake, which caused no severe structural damage, and about 500 smaller ones so far in October, have made residents such as Annamaria Scardi, a mother of two teenagers, feel tense.

'Even those small ones (quakes) make us afraid,' she told Reuters. 

'We are worried because (we are supposed to) run away. But where do we go? Where? This is the situation. We're on edge.'

The government discussed the situation last month and would order an evacuation if civil protection officials warned buildings were in danger of collapse.

Carlo Doglioni, the president of Italy's National Institute of Geophysics and Volcanology (INGV), told TGCom24: 'Considering that seismic activity has only increased in recent months, at the moment we don't see an end.

The best-case scenario is that the activity would cease, as it did after long unrest in the early 1980s, and the worst would be an eruption like the one in 1538.

On September 29, 1538, Campi Flegrei awoke with fury, producing a week-long 328-foot-high cinder cone.

Another explosion hit on October 6, killing 24 people. 

Traveling across the Atlantic is the other supervolcano that has caught attention in recent months.

Long Valley Caldera was the site of a super explosion 767,000 years ago, releasing 140 miles of volcanic material into the atmosphere and devastating the land. 

Over a year and a half, researchers at the California Institute of Technology (Caltech) used the cable to measure more than 2,000 seismic events beneath the valley, most too small to be felt by people.

However, a study in 2018 found that under the 20-mile-long and 11-mile-wide crater lies a whopping 240 cubic miles of semi-molten magma.

The staggering discovery, made by a team with the University of California and the University of Rhode Island, of the magma means that if another eruption were to happen - which experts say is unlikely anytime soon - it would lead to a massive natural disaster. 

The researchers at Caltech believe the quakes result from fluids and gases being released as the area cools off and settles down.

Ettore Biondi, who was involved with the study, told NBC News that an eruption cannot be ruled out.

'That doesn't imply that you can't have smaller eruptions, but from a supervolcanic eruption perspective, I think as of now we are on the safe side,' Biondi said.