Albert Barnes Commentary Job 9:5

"[Him] that removeth the mountains, and they know it not, When he overturneth them in his anger;" — Job 9:5 (ASV)

Which removeth the mountains - In order to show how vain it was to contend with God, Job refers to some exhibitions of his power and greatness. The “removal of the mountains” here denotes the changes which occur in earthquakes and other violent convulsions of nature. This illustration of the power of God is often referred to in the Scriptures; compare Judges 5:5; 1 Kings 19:11; Psalms 65:6; Psalms 114:4; Psalms 144:5; Isaiah 40:12; Jeremiah 4:24.

And they know not - This is evidently a Hebraism, meaning suddenly, or unexpectedly. He does it, as it were, before they are aware of it. A similar expression occurs in the Koran, “God overturns them, and they do not know it;” that is, he does it without their suspecting any such thing; compare Psalm 35:8. Let destruction come upon him at unawares, or, as it is in the Hebrew and in the margin, which he knoweth not of. Tindal renders this, “He translates the mountains before they are aware.”

Which overturneth them in his anger - As if he were enraged. There could scarcely be any more terrific exhibition of the wrath of God than the sudden and tremendous violence of an earthquake.