The subject of this verse is the people (“men”) from the first line of verse 20. Translators should make this clear. Good News Translation has done this with the word “people.” It also restructures the verse by placing the last line at the beginning.

To enter the caverns of the rocks is very similar to the first line of verse 19. In English caverns suggests large openings or caves, but the Hebrew word here refers to narrow crevices in a rock face (so Revised English Bible, New Jerusalem Bible). In any case, the idea of a hole or a place to hide is evident.

The clefts of the cliffs is a parallel to the caverns of the rocks. The Hebrew word for cleft denotes a split in a rock face. There are broken, rocky cliffs that run the length of the Jordan Valley in Israel. One possible model for this phrase is “the deep holes in the sides of the steep mountains.”

The first half of this verse may be rendered “And they will hide in the caves in the rocks and in the openings in the cliff faces” or “… in the sides of the mountains.”

From before … to terrify the earth repeats the words from verse 19. See the comments there.

An alternative model for this verse is:

• They will go and hide in holes in the mountains [or, rocks],
wanting to escape from Yahweh’s terrible punishment
and his overwhelming power,
when he appears in order to judge the world.

Quoted with permission from Ogden, Graham S. and Sterk, Jan. A Handbook on Isaiah. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here .