Pompeii is a preserved ancient Roman city in Campania, Italy. It is at the southeastern base of Mount Vesuvius (a volcano). 

Around midday on August 24, 79 CE (one thousand, nine hundred and forty seven years ago), a huge eruption from Mount Vesuvius showered volcanic debris - rock and ash - over the city of Pompeii, followed the next day by clouds of blisteringly hot gases. Buildings were destroyed, the population was crushed or choked to death, and the city was buried. 

The rock and ash protected the buried city until it was rediscovered three hundred years ago.

During a volcanic eruption, hot melted rock called magma escapes from a vent, or opening, in Earth’s surface, or crust. Magma released from a volcano is known as lava, which is 700 to 1,200 ° C in temperature.

Strong volcanic eruptions throw bits of magma into the air. These bits cool into tiny pieces of rock, called volcanic dust or volcanic ash. 

Steam and poisonous gases also escape from volcanoes. Sometimes these gases are mixed with ash and other hot debris. This mixture travels outward in destructive fiery clouds, called pyroclastic flows.

This combination of rock and ash, followed poisonous gases mixed with more ash, covered the city.