When they were uncovering the city, the excavators came across the skeleton remains of the people of Pompeii. They realized that the volcanic ash had formed a second skin around them, and so when they would find a pocket with a skeleton in it, they would pour in this mold, and out would come this dying person’s last moments. 

This was the most intense part of walking the streets of Pompeii. To see the terror, the angst, what these people look like while they are being killed by a volcano, it is like Mother Nature preserved them to serve as a warning.