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Just to the west of Naples, in Italy, is an area riddled with craters and cones. Jets of steam and sulfurous gases vent continuously from fissures and fumaroles, while the ground itself heaves, indicating that somewhere below is a supervolcano that’s growing increasingly restless.

The Phlegraean Fields, known as Campi Flegrei in Italian, first erupted some 39,000 years ago, and when it did, it was so massive that scientists believe that it triggered a climate change that drove the last of the Neanderthals to extinction. Ash falls from the eruption was detected as far away as Greenland, frozen and preserved in ancient ice shelves and glaciers. In the remote Caucasus Mountains of southern Russia, layers of ash were discovered in caves along with Neanderthal bones and artifacts.

The Phlegraean Fields were a popular holiday destination during Roman times. On the northwest shore of the Gulf of Naples, within the Phlegraean Fields, there was once a fashionable resort town named Baiae with luxurious summer villas owned by wealthy Romans including Julius Caesar, Nero, and Hadrian. A good portion of Baiae’s ruins now lies underwater when the ground sank following local volcanic activity.

Between 8000 BC and 1700 BC there were thirteen dated prehistoric eruptions and most were moderate to large in scale. In recorded history, the first time the volcano was observed erupting was in 1198. It was a small eruption that didn’t cause much damage. In 1538, the volcano erupted again over an eight-day period and deposited enough materials to create new hill—the 400-foot-high Monte Nuovo. The force of the eruption was so powerful that it knocked down trees at distances of 5 km. Furthermore, there is evidence that the ground around this region is periodically being uplifted and subsided multiple times.

This strange phenomenon was observed in the 18th century when the King Charles of Naples ordered the excavations of a Roman-era marketplace or macellum near the shores in the town of Pozzuoli. Near the bottom of the three marble columns of the macellum are bands of borings that were identified to be made by marine mollusks. But the columns were more than 20 feet above the sea level at that time. Apparently, the columns that were originally above water, got submerged to a depth of at least seven meters based on the bandings, and then reemerged at a later date.

The Macellum of Pozzuoli. The banding can be seen on the lower half of the three columns on the right of the image.

Scottish geologist Charles Lyell argued that there is a vast magma chamber deep underground that’s periodically refilled and emptied with molten rock, causing the ground above to rise and fall. This phenomenon is now known as bradyseism. We now know that the ground around Campi Flegrei was already subsiding when the marketplace was built and continued to subside until the Middle Ages when it began to rise around AD 700 to 800 AD. Then a period of subsidence followed after which the land rose again from around 1500 up to the last eruption in 1538. With the magma chamber evacuated by the 1538 eruption, the ground subsided gradually again. Over the last fifty years, the ground has been periodically rising and falling.

Recent research have shown that the periodical uplift puts the rocks under tremendous amount of stress making them more brittle and vulnerable to breaching, allowing magma to find a way to the surface. Previously it was thought that as soon as the pressure in the magma chamber decreased, the tension in the overlying rocks would decrease as well. But new simulation suggests that the rocks are never relieved of pressure. When the ground is uplifted and the rocks break, magma is injected into the cracks and solidifies, thus maintaining the surrounding rock under pressure and tension. With every phase of uplift, tension forces build up making the rocks more and more brittle.

Like the Yellowstone caldera in western United States, nobody can say when the Phlegraean Fields will erupt, but with three million people living in Naples and the surrounding area, it is something to be concerned of. An eruption at Phlegraean Fields could possibly start a volcanic winter that could imperil millions of more lives around the world.

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