Outside of the Old City of Jerusalem, there's a small valley* called the valley of hinom. The Hebrew for "valley" is gei, so this little valley is known as "GeiHinom." It's name was sometimes morphed into a Greek version, Gehenna.
* well, it's usually small. If you're trying to hike through it in the middle of the day in the middle of thesummer, after having walked all around Jerusalem, it suddenly gets very, very big. Some of my Rabbinical School classmates can verify this.
In ancient times, several cults (including, apparently, some apostate Jewish cults) used to perform child sacrifice in that valley. Needless to say, the place took on a very sinister air to the locals, who began to associate it with all bad things. In fact, it became known as the place to which bad, evil people would go when they died. Geihinom, or Gehenna, was the earliest Jewish idea of Hell.
So, if anyone ever gave you a "It'll be a cold day in Hell before..." or "I'll do that when Hell freezes over," you really should give them a call. Because, right now, as I type this, it is snowing in Hell.
p.s. that photo is not actually the right valley. But, it's the closest valley I could find a picture of.