You probably know this one: Mt Vesuvius erupted, not entirely without warning, but far more violently than anticipated, in the year 79 CE (in the reign of Emperor Titus). It is unknown how many people died – the remains of more than a thousand have thus far been discovered, but the population of Pompeii is variously estimated between 10 and 25 thousand people, with another 5000 at Herculaneum.
The eruption buried both towns in ash and mud, altered the course of the river Samo (which had flowed past Pompeii) and pushed the coastline out to sea (so that Pompeii was now inland instead of coastal). It remains the largest and deadliest of over 40 recorded eruptions of Vesuvius, although it appears that there may have been even earlier eruptions on an even greater scale.
Mt Vesuvius remains the only active volcano on the European mainland, most recently erupting on 18 March, 1944. Although the volcano has been quiet since then, it remains active, and future eruptions are regarded as inevitable.