A "supervolcano" might sound like something out of a sci-fi fantasy film, but one supervolcano lies hidden near Pompeii, Italy, where thousands were killed in 79 A.D., and it could potentially kill millions.

"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 molten "caldera" in the area west of Naples known as Campi Flegrei, told Reuters. (A caldera is cauldron formed by land that has collapsed after a volcanic eruption, according to Yahoo! News.)

"Fortunately, it is extremely rare for these areas to erupt at their full capacity, as it is extremely rare for large meteorites to hit the earth," De Natale told Reuters. "But some of these areas, in particular the Campi Flegrei, are densely populated and therefore even small eruptions, which are the most probable, fortunately, can pose risks for the population."

The looming danger calls to mind the 79 A.D. Mount Vesuvius eruption that buried the ancient Roman city of Pompeii under a thick, suffocating layer of volcanic ash.

A supervolcano has a volcanic center that has had an eruption of magnitude 8, which means the measured deposit for the eruption is greater than 240 cubic miles, according to the U.S. Geological Survey.

And the supervolcano near Pompeii is not the only one.

The Yellowstone Caldera, located under Yellowstone Lake, is the largest supervolcano in North America. California and New Mexico also have them. Other supervolcanoes around the world are located in Indonesia, Japan, and Siberia, according to the Discovery Channel. There are even significant calderas that have not been well studied in Ethiopia and Bolivia.

The real danger of a supervolcano is the ash, which -- in large enough quantity -- can collect in the atmosphere and block out the sun, causing severe global climate changes, as ABC News' Lee Dye describes:

Such an event could make thermonuclear war or global warming seem trivial, spewing untold tons of ash into the atmosphere to block sunlight. The result would be many years of frigid temperatures, wiping out millions of species. A super-volcano that erupted 250 million years ago is now believed to have created the greatest mass extinction the world has ever seen, wiping out up to 95 percent of all plant and animal species. Some renegade scientists believe it was a volcano, not an asteroid, that killed off the dinosaurs 65 million years ago.

Dye goes on to detail a new study from Vanderbilt University researchers which states that a supervolcano eruption could happen just a few hundred years after the volcano forms. Thus, working on a very "short fuse."

Back in 2005, Stephen Sparks of the University of Bristol told LiveScience of the potential doomsday effects of a supervolcano.

"In super eruptions the magma chamber is huge," Sparks said. "When the magma erupts the overlying rocks collapse into the chamber, which has reduced its pressure due to the eruption. The collapse forms the huge crater." The eruption shoots ash and chemicals into the air. "The whole of a continent might be covered by ash, which might take many years -- possibly decades -- to erode away and for vegetation to recover," he added.

Scientists in the U.S. say that the chance of another catastrophic volcanic eruption at Yellowstone is slim, with a precise yearly probability of 1 in 730,000 or 0.00014 percent, according to the USGS. Buildup preceding such a disaster could be detectable with new technology weeks or months before the action eruption.

In Italy, scientists are monitoring the conditions at Campi Flegrei and some even say that the eruptions seem to be weakening over time, according to research included in the Smithsonian.

CORRECTION: In an earlier version of this story, Vanderbilt University was misspelled.

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  • In this June 6, 2011 file photo, lightning strikes over the Puyehue volcano, over 500 miles south of Santiago, Chile. (AP Photo/Francisco Negroni, AgenciaUno, File)

  • Lava flows during an eruption of Mt. Etna volcano, near Catania, Sicily, in the early hours of Thursday, Feb. 9, 2012. The lava flowed on the snow toward a desert valley and did not pose a treat to the inhabited area. (AP Photo/Carmelo Imbesi)

  • View of the Nevado del Ruiz volcano near Villa Maria, central Colombia, Sunday, July 1, 2012. The volcano erupted, Saturday afternoon, spewing smoke and ash. While authorities ordered the preventative evacuation of some communities around the mountain but no damages or victims were reported. (AP Photo/Luisa Garcia)

  • The Fuego volcano erupts

    The Fuego volcano erupts, viewed from San Juan Alotenango municipality, Sacatepequez departament, 65 km south of Guatemala City on May 25, 2012. The volcano has begun spewing lava and columns of ash into the air, and authorities have raised the alert level in the area. (JOHAN ORDONEZ/AFP/GettyImages)

  • A man walks atop the crest of the Xilica

    A man walks atop the crest of the Xilicatzi hill in Totolac, Tlaxcala, Mexico while ash and smoke are spew from the Popocatepetl volcano on July 4, 2012. According to a report by The National Center of Prevention of Disasters (CENAPRED) during the last 24 hours the Volcano has emitted at least 15 breaths and it has been declared a yellow alert. (GUADALUPE PEREZ/AFP/GettyImages)

  • The Fuego volcano (R) seen from Siquinal

    The Fuego volcano (R) seen from Siquinala municipality, Escuintla departament, 80 km south of Guatemala City on June 21, 2012. The volcano has begun spewing lava and columns of ash into the air, and authorities have raised the alert level in the area. (JOHAN ORDONEZ/AFP/GettyImages)

  • Popocatepetl volcano spews lava, ash and steam during an eruption as seen from from Xalitzintla, Mexico, Saturday April 21, 2012. Authorities prepared evacuation routes, ambulances and shelters in the event of a bigger explosion after the volcano that looms over Mexico City emitted a low-pitched roar early Saturday morning. Popo, as it's commonly known, has put out small eruptions of ash almost daily since a round of eruptive activity began in 1994. A week ago, the eruptions started growing larger. (AP Photo/Marco Ugarte)

  • A plume of ash and smoke rises from the Popocatepetl volcano seen from the outskirts of the town of Santiago Xalizintla, Mexico, Friday, April 20, 2012. Authorities prepared evacuation routes, ambulances and shelters in the event of a bigger explosion after the volcano that looms over Mexico City emitted a low-pitched roar early Friday morning and spewed ash and steam. (AP Photo/Gonzalo Perez)

  • This natural-color satellite image provided by NASA was acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra satellite on Saturday June 11, 2011 of the Puyehue-Cordon Caulle eruption in Chile. In the image, the ash plume from the volcano is visible center. Some airline flights to and from New Zealand were canceled Sunday as the ash cloud from the erupting volcano in southern Chile spread. (AP Photo/NASA)

  • A snowcapped mount Etna erupts not far from Zafferana Etnea village, in Italy, in this frame from video Thursday, April 12, 2012. The south-eastern crater, ┬"born┬" in 1971, has been the most active in the last few years. In the recent past lava flows have mainly damaged properties, but due to its slow speed lava has not killed human beings. So far Italian authorities have not issued any warning of dangers for houses and people. (AP Photo/APTN)

  • Ashes and smoke billow through the clouds after the eruption of the Puyehue-Cordon Caulle volcano in Chile, seen from the border cross with Argentina, Cardenal Samore, in southern Chile, Wednesday June 15, 2011. The volcano started erupting on June 4 after remaining dormant for decades. The ash spread across the Pacific, prompting authorities to suspend flights in Australia and New Zealand. (AP Photo/Alvaro Vidal)

  • In this Sept. 21, 2011 photo provided by the U.S. Geological Survey, lava flows from Kilauea volcano at Volcanoes National Park in Hawaii. Lava began flowing from the fissure breakout that started last week at Kilauea, a volcano that has been continuously erupting since 1983. It advanced about 2.3 miles before stopping short of the mostly abandoned Royal Gardens subdivision. (AP Photo/USGS, Tim Orr)

  • A plume of ash, dust and steam emits from a volcano erupting beneath Iceland's Eyjafjallajokull glacier, in this file photo dated Wednesday, April 21, 2010, in Eyjafjallajokull, Iceland. A surge of small earthquakes has been reported Tuesday Sept. 6, 2011, around Iceland's Katla volcano, but scientists said Tuesday there is no immediate concern that the increased seismic activity will trigger a dangerous eruption, although history has shown that when the Eyjafjallajokull volcano erupts, Katla isn't far behind. (AP Photo/Carolyn Kaster, File)

  • In this photo provided by the regional government of Aysen, plumes of smoke and ash rise from the Hudson volcano in the region of Aysen, Chile, Thursday Oct. 27, 2011. The Southern Andean Volcano Observatory says seismic activity related to the volcano increased starting early Tuesday night. The Hudson volcano lies 995 miles (1,600 kilometers) south of Chile's capital, Santiago. It last erupted in August 1991, causing millions in damages to local farms and killing an estimated 1.5 million sheep. Chile has about 3,000 volcanos, 500 of which are considered geologically active. (AP Photo/Aysen Regional Government)

  • The Tungurahua volcano spews ashes during an explosion as it is seen from Cotalo, Ecuador, Tuesday, Nov. 29, 2011. Ecuador's Geophysical Institute says increased activity that began Sunday is billowing columns of ash, sending superheated clouds of gas down the slopes and cascading hot rocks from the summit. (AP Photo/Dolores Ochoa)

  • In this photo released by Virunga National Park Monday, Nov. 14, 2011, Mount Nyamulagira in eastern Congo is seen erupting Friday, Nov. 11, 2011. Virunga National Park in Congo is inviting tourists on an overnight trek to view a spectacular eruption of Mount Nyamulagira, where rivers of incandescent lava are flowing slowly north into an uninhabited part of the park but pose no danger to its critically endangered mountain gorillas. (AP Photo/Virunga National Park, Cai Tjeenk Willink)

  • A view of the Tungurahua volcano as it erupts as seen from Cotalo, Ecuador, Wednesday, Nov. 30, 2011. Ecuador's Geophysical Institute said the volcano's increased activity that began Sunday, is billowing columns of ash, sending superheated clouds of gas down the slopes and cascading hot rocks from the summit. (AP Photo/Dolores Ochoa)