According to the Old Testament, the Valley of the Son of Hinnon (Hebrew: גיא בן הינום) is one of the two principal valleys surrounding the Old City.
The Valley is first mentioned in the Book of Joshua 15:8-
"וְעָלָה הַגְּבוּל גֵּי בֶן-הִנֹּם, אֶל-כֶּתֶף הַיְבוּסִי מִנֶּגֶב--הִיא, יְרוּשָׁלִָם; וְעָלָה הַגְּבוּל אֶל-רֹאשׁ הָהָר, אֲשֶׁר עַל-פְּנֵי גֵי-הִנֹּם יָמָּה, אֲשֶׁר בִּקְצֵה עֵמֶק-רְפָאִים, צָפוֹנָה"
"And the border went up by the Valley of the son of Hinnom unto the side of the Jebusite southward--the same is Jerusalem--and the border went up to the top of the mountain that lieth before the Valley of Hinnom westward, which is at the uttermost part of the vale of Rephaim northward."
The Valley of the son of Hinnom, Gai Ben-Hinnom, is a term derived from the Hebrew Bible which describes Gehenna (Hebrew: גהנום) literally means Hell. In the Talmud the valley is called Gehinnom.
Why is this valley called Hell? The original Hebrew word, pronounced hin-nowm, means a cry of sorrow and grief. The Valley of Hinnom had a very horrendous History in ancient times. It was used as a place where Sodomites and other idolaters did all sorts of vile and wicked things, including burning children alive as sacrifices to the idols Moloch and Baal. One section of the valley was called Tophet, or the fire-stove, where the children were slaughtered.
After their return from the Babylonian exile, the people of Judah turned the Hinnom Valley into the city dump where garbage and anything deemed unclean was incinerated. For that purpose, a fire was kept constantly burning there. Even though it was no longer used for evil worship, with all the filth and thick smoke it remained a very dark and dreary place.