Albert Barnes Commentary Job 38:18

"Hast thou comprehended the earth in its breadth? Declare, if thou knowest it all." — Job 38:18 (ASV)

Have you perceived the breadth of the earth? — How far the earth extends.

To understand the force of this, we must remember that the early conception of the earth was that it was a vast plain, and that in Job’s time its limits were unknown.

One of the earliest and most obvious inquiries would naturally have been: What was the extent of the earth? By what was it bounded? And what was the character of the regions beyond those then known?

All this was hidden from humanity at that time. God, therefore, asks with emphasis whether Job had been able to determine this great inquiry. The knowledge of this is put on the same foundation as that of the depths of the sea and of the dark regions of the dead; in Job’s time, the one was as much unknown as the other.

God, who knew all this, must, therefore, be infinitely exalted above humanity.