Where did the matter from the decomposed Pompeiian bodies go? (Conservation of matter) After the (in)famous eruption of Mt. Vesuvius, Pompeii was buried under ash.  In 1860, an archaeologist digging down found cavities where a body had been, and injected plaster.
I understand that that the bodies would decompose, but all of the matter should still be there, right?  Conservation of matter, right?  How can there be a hollow space where the body was?
 A: This is really more about archaeology, biology, and/or chemistry than physics. Of course mass-energy is conserved. That's not the issue.
Compacted lava ash is porous. Liquids and maybe even microbes can squeeze through it.
Except for bones, everything in a human body is biodegradable and/or liquid. And bones can dissolve too, given enough time.
In other words, the body shaped spaces in Pompeii were not mysterious voids of vanished mass. They were filled with leftover gases, some bone fragments and maybe dessicated tissue, after the rest of the victim's decomposed mass drained and evaporated away. 
Ok, that's probably close to the right answer. I'm not a Pompeii expert. An archaeologist or forensic MD would be able to explain it better.
In general, when any topic doesn't make sense, a curious person should ask "what knowledge am I missing about this subject area?" many times before considering "what if the laws of physics are wrong?"
A: The bodies didn't go anywhere.   The ash cast of the bodies still contain the bones of the victims.  The pyroclastic flows scorched the the lungs of the bodies and seriously burned the bodies.  
As the bodies decomposed, they left a void in the ash cast.......
