Fast-moving lava hit the citizens of Pompei quickly. They were enveloped by the flow, which quickly cooled and hardened. As it did, the bodies of those who fell victim to this historic catastrophe inevitably decomposed, leaving cavities in the now newly-formed rock. Pouring plaster into these cavities produced some rather compelling statues of the victims. Arguably the most authentic form of art you can find. Certainly the most tragic.