Al-Fajr: (21) Nay, when the earth is leveled…

"Nay" (Kalla) is a rebuke to them for that. His statement, the Exalted, "When the earth is leveled, leveled," and what follows, is a resumption of speech brought forth by way of threat, serving as an explanation for the rebuke.

Regarding ad-dakk (leveling/crushing), al-Khalil said: it is the breaking of a wall, a mountain, and the like. Its repetition is for the purpose of indicating total encompassing; thus, the second is not an emphasis of the first, but rather it is similar to the case in your saying, "They came man by man," and "I taught him arithmetic chapter by chapter." That is, when the earth is leveled, leveled successively, until it is broken and everything upon its surface—mountains, buildings, palaces, and others—has departed, when it is shaken time after time and becomes scattered dust.

Al-Mubarrad said: ad-dakk is the lowering of that which is raised by leveling and smoothing. It is said, "The hump of the camel is leveled (nudikku)" when it collapses into its back, and a she-camel is dak-ka (flat-backed) if she is such. The meaning according to this is: when it is leveled, leveling after leveling, and nothing remains upon its surface until it becomes like a smooth rock. Whatever the case may be, it is, as has been said, an expression for what befalls the earth at the second blowing of the Trumpet.