The Valley of Hinnom had a very horrendous history in ancient times. It was used as a place where the pagan worshipers 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 (2 Kings 23:10). It was a place of tremendous evil for many years.
After their return from the Babylonian exile (see Why Babylon?), the Jews 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 traditional concept of hell originated. By the time of Jesus Christ, the deep, constantly-burning Valley of Hinnom was also known as the Valley of Gehenna, and had taken on a popular image as the place "down there" (i.e. in the valley) where the wicked would eventually be cast into the flames for destruction.
The Lake Of Fire - The Real "Hell"
"And I saw the beast and the kings of the earth with their armies gathered to make war against him who sits upon the horse and against his army. And the beast (see The Antichrist and The Lawless One) was captured, and with it the false prophet (see The Great False Prophet) who in its presence had worked the signs by which he deceived those who had received the mark of the beast and those who worshiped its image. These two were thrown alive into the lake of fire that burns with sulphur." (Revelation 19:19-20 RSV)
"Then I saw a great white throne and him who sat upon it; from his presence earth and sky fled away, and no place was found for them (see The Last Day). And I saw the dead, great and small, standing before the throne, and books were opened. Also another book was opened, which is the book of life. And the dead were judged by what was written in the books, by what they had done. And the sea gave up the dead in it, Death and Hades gave up the dead in them, and all were judged by what they had done. Then Death and Hades were thrown into the lake of fire. This is the second death, the lake of fire; and if any one's name was not found written in the book of life, he was thrown into the lake of fire." (Revelation 20:11-15 RSV)
Fact Finder: Will the surface of the earth one day be "flooded" with fire?
2 Peter 3:5-7,10-11
See also Planet On Fire!