Musar zu Dewarim 32:8

A different approach to the meaning of the expressions אימה, חשיכה, גדולה-נופלת, is taken by the Arizal, who sees in אימה, an allusion to the generation of Enosh, during whose lifetime one third of the earth was flooded. G–d had done this in order to inspire fear and reverence for Him in mankind, hence the word אימה, fear. חשיכה refers to the generation of the deluge, when according to our sages the luminaries ceased to function, hence the word חשיכה, darkness (Yerushalmi Pesachim 1,1). The word גדולה refers to the generation of the Tower, i.e. מגדל, and the word נופלת refers to the destruction, i.e. תהפוכה of Sodom whose inhabitants were destroyed body and soul. What Abraham experienced was the awareness that these four events represented delays in G–d's plan for His universe, and that the earth needed תקון, a reformation in order to get back on the "right track." Abraham was seventy years old when He received G–d's first assurance that his descendants would possess the Holy Land. An allusion to this is found in the verse יצב גבולת עמים, "He establishes the boundaries of the nations" (Deut. 32,8). In that verse Israel is contrasted with the other seventy nations.