What exactly is "Emek ha'Sidim"?
Rashi and Targum Onkelus: A valley comprising many fields (or orchards
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 S'dom and Amora.
Targum Yonasan: The aforementioned valley sprouted pools of freshwater which then flowed into the Yam ha'Melach. 3
Though this itself, is unclear.
Gur Aryeh: Rashi adds this explanation because the words "Hu Yam ha'Melach" imply that the area itself became Yam ha'Melach.
See Na'ar Yonasan who explains this allegorically - describing the deeds of the Sodomites, which were initially praiseworthy but later turned out to be rotten, and to their territory, which was initially fruitful, but ultimately turned into the Salt Sea.