What exactly is "Emek ha'Sidim"?
Rashi and Targum Onkelus: A valley comprising may fields (or orchards - Targum Yonasan).
If it was a 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.
Targum Onkelus: It was the current location of the Yam ha'Melach, that was destined to form after the destruction of Sedom and Amora.
Though this itself, is unclear.
רש"י: ומדרש אגדה וכו': למה הביא את המדרש?
גור אריה: פשטות לשון הכתוב שעמק השדים עצמו הוא ים המלח, ולכן צ"ל שנתבקעו הצורים וכו'.