Albert Barnes Commentary Isaiah 34:3

"Their slain also shall be cast out, and the stench of their dead bodies shall come up; and the mountains shall be melted with their blood." — Isaiah 34:3 (ASV)

Their slain also shall be cast out - They would lie unburied. The slaughter would be so extensive, and the desolation would be so entire, that there would not remain enough to bury the dead (compare the notes at Isaiah 14:19).

And the mountains shall be melted with their blood - The expression here is evidently hyperbolical, and means that as mountains and hills are wasted away by descending showers and impetuous torrents, so the hills would be washed away by the vast quantity of blood that would be shed by the anger of Yahweh.