The Son Of Hinnom
The Hinnom Valley is a deep, narrow ravine located in Jerusalem, running south from the Jaffa Gate on the west side of the Old City (see also Physical and Spiritual Gates), then eastward along the south side of Mount Zion (seen in the top of the photograph with the Hinnom Valley below it - see also The Zion Bridge and Who, What or Where Is Zion?) until it meets the Kidron Valley which separates the Temple Mount from the Mount of Olives on the east side of the city. It is named from a certain "son of Hinnom" who apparently owned or had some significant association with the valley at or before the time of Joshua. It is later (see quotes from 2 Kings 23:10 below) referred to as the "valley of the children of Hinnom" (KJV)
"15:8 And the border went up by the valley of the son of Hinnom unto the south side of the Jebusite; 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 end of the valley of the giants northward" (Joshua 15:8 KJV)
"The valley of the children of Hinnom"
Coincidentally or not, 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. Josiah, a relatively righteous king of Judah, put a stop to it, at least temporarily.
"23:7 And he brake down the houses of the sodomites, that were by the house of the LORD, where the women wove hangings for the grove. ... 23:10 And he defiled Topheth, which is in the valley of the children of Hinnom, that no man might make his son or his daughter to pass through the fire to Molech." (2 Kings 23:7,10 KJV)
After their return from the Babylonian exile (see Why Babylon?), the people of Judah turned the Hinnom Valley into the city dump where garbage and anything deemed unclean (including the bodies of executed criminals) 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.
The Hebrew name Hinnom when translated into Greek is gehenna, from which the translated word and concept of a burning "hell" originated (there is also another "hell" - see the Fact Finder question below). By the time of Jesus Christ, the deep, constantly-burning Valley of Hinnom was also known as the Valley of Gehenna, or Hell, and had taken on a popular image as the place "down there" where the wicked would eventually be cast into the flames for destruction.
Fact Finder: What "hell" has no fire?