Our current perceptions of Hell derive primarily from two sources: John Milton's Paradise Lost and Dante's Inferno. However, hundreds of years prior to these great works, there was Gehenna, a truly loathsome place of never-ending fire and noxious fumes that became a living metaphor for Hell on Earth.
The name Gehenna is a derivation of the Valley of Hinnom, a ravine outside of old Jerusalem. Starting somewhere around 800 BC, some of the inhabitants of Judah used the valley as a place to worship Moloch, an ancient Ammonite deity. The details of the worship are pretty gruesome, so if you are squeamish, stop reading.
Followers of Moloch erected a bronze statue, depicting the god as having a bull's head and outstretched arms. The statue was hollow or had some sort of fire pit in it. Worshippers would place children to be sacrificed in Moloch's arms, where the flames would gradually consume them until they fell into the pit. There are descriptions of the participants banging drums during the ceremony to drown out the child's screams, but these may be an attempt by subsequent conquerors to make Moloch's followers seem more barbaric than they actually were. But, even so, it's all pretty despicable.
Josiah, who became king of Judah in 641 BC, thought so too. He destroyed the statue of Moloch, and "defiled" (the word used in the Bible) the Valley of Hinnom. The popular notion is that he turned it into a huge garbage dump that burned day and night, for the symbolic purpose of cleansing the valley of its sins, but also for the practical purpose of getting rid of Jerusalem's trash. That trash included the bodies of criminals and other low-lifes who didn't warrant a proper burial. Thus the Valley of Hinnom first became a site of pagan worship and child sacrifice, and then became Gehenna, a place of eternal flame and stench where sinners went after they died...Hell on Earth.
Is This for Real?
There are many references to Gehenna as Hell in the Christian Bible, but also in the Hebrew Bible and the Koran. It was a popular metaphor. But maybe people were just referring to the Tophet ("roasting place") where the actual sacrifices took place, and not to the burning trash heap. No one really knows. The story of Moloch-worshippers and child sacrifice is pretty well-established, as is Josiah's ending the practice. Josiah purging the valley by dumping garbage in it and setting it on fire makes some sense, but there is no real archaeological evidence to support or refute it.
The vivid imagery of the story has survived. A quick dA search for "Gehenna" turns up some characters of that name, some half-naked women (what a surprise!), and a song by slipknot. My favorite however, is this quote from EA Poe's story Morella:
"And thus, joy suddenly faded into horror, and the most beautiful became the most hideous, as Hinnom became Gehenna."
So there you have it. If you, like me, have seen references to Gehenna and wondered what it was, now you know.