I went down the crater of Mount Eden (Maungawhau) when it was still ok, in the 70s (it's considered sacred now). Back then it was full of tennis-ball-sized lumps of red scoria (now grassed over). Creepy feeling to be at the bottom of a crater which you knew they were taking the temperature of every day, looking up at the rim all round you and imagining if you could run for it if it went off.

As I understand it, the whole Bay of Naples and quite a bit beyond is one huge caldera, generally called the Campi Flegrei after one small bit of it. Lake Taupo in NZ is even bigger. On the road round the lake you can look round and contemplate the fact that not only is the whole lake (still steaming in places, 26,000 years on) part of the caldera, so is everything you can see as far as the horizon, maybe 30 miles away.

Current thinking is that we would get a few centuries warning if something on that scale was going to go off (there are several candidates). Those years would be an interesting bit of human history.