Albert Barnes Commentary Job 28:5

"As for the earth, out of it cometh bread; And underneath it is turned up as it were by fire." — Job 28:5 (ASV)

As for the earth, out of it cometh bread - That is, it produces food, or the materials for bread. Job's idea seems to be that it was proof of great wisdom and skill on man's part that he had advanced the arts of agriculture so far. The earth producing grain, and the skills of farming, were illustrative of wisdom and skill, but they did not impart the wisdom about God's government that was desired. That was reserved to be imparted more directly by God Himself (Job 28:23 and following).

And under it is turned up as it were fire - That is, when turned up, it discloses precious stones that seem to glow like coals of fire. This is the obvious sense of this passage, although a different interpretation has been given by most expositors.

Job is speaking of mining. He describes the search for gold, silver, and precious stones. He says that one of the wonders of wisdom in the earth is that it produces nutritious grain; another is that when the same earth is turned up, it seems to rest on a bed of fire. The dark ground is made to glow by the quantity of jewels that are disclosed, and its deep recesses seem to be on fire.

Therefore, it seems to me, there is no reference here to any volcanic agency or to any belief that the earth rests on a sea of fire. The idea has been expressed in Sergeant’s “Mine:”

“Wherever our footsteps turn,
Rubies blush and diamonds burn.”

Luther has given the passage a different meaning. Man bringet auch Feuer unten aus der Eerie, da oben Speise auf wachst - “They bring fire from the earth beneath, where food grows up above.” Coverdale: “He bringeth food out of the earth; that which is under he consumeth with fire.” Herder: “And underneath it is changed as by fire.” Dr. Good: “Below it (the earth) windeth a fiery region.”