“The waters prevailed so mightily upon the earth that all the high mountains under the whole heaven were covered.”
This is a notice not merely respecting the great extent of the Flood, but the action of ‘covering’, much like that of ‘blotting out’ (cf. Psalm 51, the “Miserere”), carries with it sacrificial and expiatory connotations. Mountain tops are places of communication with the divine, and these had been defiled by the corruption of flesh. Perhaps they were even places where false gods had received sacrifice. By covering them all, God wipes away the stain of this corruption.