The town of Pozzuoli is a workaday sort of place, but at the bottom of the town is the Macellum, or market place, which has three upstanding marble columns which have signs of erosion caused by molluscs a long way up them, showing that the ground level was much lower in Roman times, another example of the bradyseism I mentioned. It's a bit of an uphill slog from there, passing some unexcavated Roman remains below the road at one point, until you reach the Solfatara crater. In 2013 it was €6 to go in, but you can bet it's gone up since then. The crater floor is flat and covered in a stark white deposit. There are plenty of menacing fumaroles spouting sulphureous fumes accompanied by loud hissing. There are also areas of boiling mud pools, one of which was the scene of a tragedy in 2017 when a young boy and his parents died - the lad fell in and the parents died trying to rescue him. In the early 4th century San Gennaro, patron saint of Naples, and San Proculus, patron saint of Pozzuoli, were both beheaded in the Solfatara. You can still see San Gennaro's bones in the crypt of the duomo in Naples (if you really want to). There was a phreatic eruption (ground water reacting with the underlying magma chamber) in 1198, so you wonder whether the volcano is flexing its muscles...

There are two amazing Roman villas in nearby Stabiae (where Pliny The Elder died during the 79 AD eruption of Vesuvius). If you take a look at the satellite photo on wiki you can clearly make out the circle of the caldera, and you can see the little white patch which is the Solfatara crater. And plenty more. I don't half rattle on a lot about that area, but it's my absolute favourite place in the whole world. Sorry about that!