Also, there's a phial of San Gennaro's blood in the Duomo that "miraculously" liquefies three times a year, and many a devout Catholic turns up to pay tribute...yikes...

We visited the Solfatara crater in 2013, a sinister, primeval sort of place with boiling mud and hissing fumaroles. Gennaro was beheaded there in 303. The crater last erupted in 1198 when groundwater reacted with shallow magma, a phreatic eruption in other words. Sadly, a family of three died in boiling mud a couple of years ago and I'm not sure that it's so easy to visit the crater any more. When we were there we did think it was rather too easy to put yourself at risk... Unusually, it isn't situated high on volcanic slopes. you just stroll in through a rather grand entrance, having paid your several euros first of course!