In the Old Testament, the Hinnom Valley or Gehenna was a place of abominable pagan rites, including infant sacrifice. It was there that the apostate kings Ahaz and Manasseh made their children “pass through the fire” to the god Molech. The rites were specifically celebrated in Tophet, the “place of abhorrence,” one of the chief groves in the valley
King Josiah of ancient Judah finally put an end to these abominations. He defiled the valley, rendering it ceremonially unclean (II Kings 23:10). Later the valley became the cesspool and city dump of Jerusalem - a repository for sewage, refuse and animal carcasses. The bodies of despised criminals were also burned there along with the rubbish. Fires burned continuously, feeding on a constant supply of garbage and refuse.
Aceldama, the “field of blood”, purchased with the money Judas received for the betrayal of Christ (Matt. 27:8) was also in part of the valley of Hinnom.
That is where Jesus used the illustration of hell and possibility could have been near the area when he was talking to his disciples about hell...