The plaster cast figures from Pompeii are pretty famous. The casts are made by pouring plaster into hollows left in the ash by an item that has decomposed (like bodies, wood, etc.). The resulting figures capture the expressions and last moments of the people who were killed by the eruption of Vesuvius in 79CE.
A snippet of a documentary shows the mice from Pompeii captured using the same plaster technique developed by Fiorelli:
On a side note, it’s interesting to read the recounting of the eruption in letters from Pliny the Elder to Tacitus.