Adam Clarke on Job 9:5

Removeth the mountains, and they know not - This seems to refer to earthquakes. By those strong convulsions, mountains, valleys, hills, even whole islands, are removed in an instant; and to this latter circumstance the words, they know not, most probably refer. The work is done in the twinkling of an eye; no warning is given; the mountain, that seemed to be as firm as the earth on which it rested, was in the same moment both visible and invisible; so suddenly was it swallowed up.