Report on the BBC about vulcanologists looking at Mount Vesuvius, which has 3 million people living within 20km.
Apparently, the eruption that destroyed Pompeii in 79AD may have named the type of eruption (Plinian), but it wasn't that big. Not compared to a stone age eruption they have been investigating
"Within the city of Naples they found deposits of this ash up to 3m (10ft) deep".
Now, admittedly you should get more warning than the US did about Katrina hitting New Orleans, but still - that's three million refugees. Who won't want to go back to homes that are 3 metres under ash.