The short answer to this question is that the Vesuvius eruption completely destroyed the city of Pompeii and nearby Herculaneum, killing thousands of inhabitants. Most of the people in the cities were killed by inhaling the toxic fumes emitted by the eruption, in particular carbon monoxide. But they were also pummelled by pumice stones and cinders, and covered with a thick layer of volcanic ash. It was precisely this layer of ash and mud, of course, that preserved Pompeii almost as it was at the moment of the eruption. When the city was first excavated in the mid-eighteenth century, it was thus a major archaeological find, and it remains one that still yields important information about life in an ancient Roman town.