The eruption of Mount Vesuvius near Naples, Italy, in 79 A.D. killed thousands of Pompeii residents and remains one of the most famous volcanic explosions in history. But scientists say a hidden "super volcano" in the same area has the potential to 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 project to drill deep under the earth to monitor the molten caldera, told Reuters.

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A caldera, or cauldron, is formed by collapsed land after a volcanic eruption. It can be just as dangerous as volcanic domes, sending magma and ash shooting into the air. A caldera is located in Campi Flegrei. The regional park, which is named after the Greek word for burning, is a major tourist attraction, and the surrounding area is home to more than 3 million residents.

"That is why the Campi Flegrei absolutely must be studied and monitored," De Natale said. "I wouldn't say like others, but much more than the others exactly because of the danger given that millions of people live in the volcano."

Scientists funded by the International Continental Scientific Drilling Programme have been given clearance to drill 2.2 miles underground into the center of the caldera, home to a giant chamber of molten rock. Once they've reached the chamber, they plan to install a monitoring system that would give advanced warning of any potentially dangerous eruptions.

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"[S]ome 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," said De Natale, from the Vesuvius observatory at Italy's National Institute for Geophysics and Volcanology.

The initial stages of drilling have already turned up some scientific evidence, including samples of volcanic rock from a major eruption that occurred some 15,000 years ago.