We recently found out that Gehenna was actually a place in Jerusalem that is known because there were made child sacrifices in there, we also read that probably was a area where the outcasts and infected were dumped or maybe just where the garbage were burnt. Anyway our first idea is related to the Jewish and Christian analogy of a lake of fire or Hades/hell, a place of punishment called Gehenna. It is curious that the word Hell wasn’t in the Bible until the King James version (XVII century), Gehenna was the original word used. Like we said before, probably was an analogy to a horrifying and stinky place.
This design shows the souls of the condemned struggling to reach the heaven, theirs bodies are snake-like as we see described in some parts of Divine Comedy, they are cursed to crawl for all eternity and that was their punishment.