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Campi Flegrei, Italy
There are two problems going on here. First, it’s a caldera that is feeling a tad restless. Second, it sits within an area with a population of 6 million. So if it suddenly goes BOOM, a whole lot of people are going to suffer the consequences. The last time it erupted was in 1538 and while it was relatively uneventful, it’s due for something big.