I've always been very interested in the area near Naples known as Campi Flegrei, or the Phlegraean Fields, which is a caldera eight miles across, largely submerged in the Bay of Naples but with some manifestations on land, the Solfatara volcano for example. There was a huge supereruption of Campi Flegrei around 39000 years ago which measured 7 on the VEI scale (which basically means a very big eruption, the biggest in Europe for 200,000 years). Very shortly after the eruption there was a sharp climatic cooling, and at the same time the Neanderthals died out in Europe, to be replaced by modern humans. There are notions afoot, not settled by science, that these events were linked to the eruption. The caldera is still active, with a shallow magma chamber "hotspot" that was recently shown to be linked to the one under Vesuvius, about 12 miles away. The Solfatara crater is full of boiling mud pools and fumaroles, a great place to visit (there's a nice café/bar at the entrance and a nearby campsite that provides smelly nights). The area is noted for its rather scary bradyseism, large areas of land rising and falling in response to the magma underneath shifting around. In the early 80s the town of Pozzuoli was uplifted very sharply over a couple of years by several feet, and was evacuated for fear of an eruption. That time all was well. Like Yellowstone, it's a place worth keeping a wary eye on.