The traditional southern boundary of the city of Jerusalem is the Hinnom Valley. If you are a student of the Bible at all, then you can probably list a number of times that it is referenced. While serving as boundary between the tribes of Benjamin and Judah, it also is the location of many terrible acts of pagan worship. The Hinnom Valley comes through the western side of the city before turning east and merges into the Kidron Valley. The confluence of the Hinnom and Kidron Valleys is very near the location of En Rogel, which we read about in 1 Kings 1.
Yesterday, I read a really good article by Kyle Pope from Focus Online about the Hinnom Valley. I invite all of you to read it as well.