What exactly is "Emek ha'Sidim"?
Rashi and Targum Onkelus: A valley comprising many fields (or orchards - Targum Yonasan).
If it was a fertile valley, how can the Pasuk refer to it as "the Yam ha'Melach"?
Rashi #1: At a later date, the Yam ha'Melach flowed into it. 1
Rashi #2 (citing the Midrash): Surrounding rocks split up and canals flowed from them into the erstwhile valley. 2
Targum Onkelus: It is the current location of the Yam ha'Melach, which was destined to form after the destruction of Sedom and Amora.