All of the rubbish from the entire city of Jerusalem was dumped and burnt here.
Otherwise called "the valley of the son of Hinnom," or "the valley of Benhinnom"; a deep and narrow ravine with steep, rocky sides to the S and W of Jerusalem, separating Mt. Zion to the N from the "Hill of Evil Counsel," and the sloping rocky plateau of the "valley of Rephaim" to the S. The earliest mention of the valley of Hinnom is in Josh 15:8; 18:16, where the boundary line between the tribes of Judah and Benjamin is described as passing along the bed of the ravine. On the southern brow, overlooking the valley at its eastern extremity, Solomon erected high places for Molech (1 Kings 11:7), whose horrid rites were revived from time to time in the same vicinity by the later idolatrous kings. Ahaz and Manasseh made their children "pass through the fire" in this valley (2 Kings 16:3; 2 Chron 28:3; 33:6), and the fiendish custom of infant sacrifice to the fire-gods seems to have been kept up in Topheth at its southeast extremity for a considerable period (Jer 7:31; 2 Kings 23:10). To put an end to these abominations the place was polluted by Josiah, who rendered it ceremonially unclean by spreading over it human bones and other corruptions (2 Kings 23:10,13-14; 2 Chron 34:3-5). From that time it appears to have become the common cesspool of the city, into which its sewage was conducted to be carried off by the waters of the Kidron, as well as a laystall, where all its solid filth was collected. From its ceremonial defilement and from the detested and abominable fire of Molech, if not from the supposed everburning funeral piles, the later Jews applied the name of this valley Ge Hinnom, "Gehenna," to denote the place of eternal torment. The name by which it is now known is Wadi Jehennam, or Wadi er Rubeb. See Gehenna; Hell.
THE VALLEY OF HINNOM (GEHENNA)
All the rubbish of Jerusalem was burnt here. Its interesting to note that Aceldama (Field of Blood) was here in this valley.
First Century Jerusalem
Bible History Online