Pompeii, Italy

So, when the irruption happened in 79 A.D., it happened to a city that was not near its maximum population. An interesting note, there was no term for volcanoes during that time, not to mention the people had never seen anything like that; they didn't know what was going on when Vesuvius started to erupt.
When Vesuvius started to erupt, it pushed pumice and ash into the air (estimated height of 20.5 miles), it came down in the span of several hours. It buried the city of Pompeii (and neighboring towns) in several meters of pumice and ash (Pompeii is 5.85 miles and Herculaneum is 4.25 miles from the caldera of Mount Vesuvius). Even though the ash and pumice didn't weigh very much, the sheer volume of it caused a lot of the structures to collapse. Ceilings fell into second floors, second floors fell into the ground and so on. When people were trying to wait this out, hoping that it went away, many of them were crushed or buried in their houses. Others, who could stay on top of the ash and pumice got killed from the pyroclastic flow. The pyroclastic flow is a dense wave of superheated air and rock that moves from the erupting volcano. Its velocity is such, that it can cross thousands of square miles of land quickly. This superheated wind essentially vaporized these people instantaneously. In all, an estimated 16,000 citizens perished.
Moving on to those who were caught under the pumice and ash. Over time, water and rain will slowly trickle down through the pumice and ash, converting these substances to something similar like concrete. These people wear encased. When excavation started they found these cavities in certain areas. These cavities were once where something alive was, either a human or animal. Over the course of several hundred years these bodies decomposed leaving and empty cavity and sometimes bones. When discovered, these cavities were filled with plaster. This plaster made a mold of the position the individual or animal was in when it died. That's what the plaster forms of people and animals come from.
The only reason they knew the location of Pompeii, was because of the indentation where the theater was at; keep in mind, this area was completely covered. Also, the lava flow caused Pompeii to no longer be a city on the sea. The eruption ended the ancient city of Pompeii. On a final note, Pompeii was not the only city affected by Vesuvius, the city of Herculaneum was also destroyed by the volcanic pyroclastic blast and lava.