Gehenna is so closely identified with the idea of hell in the scriptures that is is sometimes simply translated as "hell."
Matthew 10:28 Do not fear those who kill the body but cannot kill the soul; rather fear him who can destroy both soul and body in hell.
But Gehenna is the Aramaic rendering of the Hebrew "Geh Hinnom" or Valley of Hinnom.
It doesn't look so bad. No lakes of any sort, certainly no lakes of fire.
According to Joshua 15:1-12, the valley marks the boundary between the land given to the tribes of Judah and Benjamin (18:1-20) and was already named for the son of Hinnom at the time the Israelites entered Canaan. It was simply another piece of the promised land.
There were a few notorious events involving fire and the valley:
2 Chronicles 28:3 King Ahaz made offerings in the valley of the son of Hinnom, and made his sons pass through fire, according to the abominable practices of the nations whom the God-of-Sinai drove out before the people of Israel.
2Chronicles 33:6 King Manasseh made his son pass through fire in the valley of the son of Hinnom, practiced soothsaying and augury and sorcery, and dealt with mediums and with wizards.
Jeremiah 19:6 Therefore the days are surely coming, says the Living-God, when this place shall no more be called Topheth, or the valley of the son of Hinnom, but the valley of Slaughter.
Perhaps we're reading the wrong (or at least incomplete) Jewish scriptures.
The Greek speaking Jews of North Africa whose complete scripture collection became the bible of the early church included both the books of Enoch.
Enoch 10:13 In those days they shall be led off to the abyss of fire: and to the torment and the prison in which they shall be confined forever.
Enoch 100:9 Woe to you, sinners, on account of the words of your mouth,
And on account of the deeds of your hands which your godlessness as wrought,
In blazing flames burning worse than fire shall you burn.
And, in the Greek Jewish scriptures is an expansion of the books of Ezra and Nehemiah, 1 and 2 Esdras. This is the first place that the lovely Valley of Hinnom is set on fire and filled with sinners:
2 Esdras 7:36 The pit of torment shall appear, and opposite it shall be the place of rest; and the furnace of hell (gehennae) shall be disclosed, and opposite it the paradise of delight.
How strange that this shifting notion has become a bedrock fundamental of some people's (and some church's) theology.
So where is hell? The beautiful and over populated Valley of Hinnom outside of Jerusalem? The center of the sun?
And what does it mean that the idea of hell grew legs and wings in the scriptures (including those that most neglect) and has taken flight to the outermost realms of the theological imagination?
The idea and location of heaven is much simpler: wherever God is, above and beyond this world. This strikes me as somewhat bipolar.