Pompeii was an ancient city in Italy, famous for being covered by the ash of the eruption of Mount Vesuvius.
When Mount Vesuvius erupted in 79 AD, the ashes quickly spread throughout the area, and the burning ash fell from the sky and coated the entire city in several layers. The results can be seen in the ruins of the city; the ash was so hot and thick, when it landed it kept the shape of the items it touched (famous portions include a dog tied to a pole and a couple kissing).