The Psalmist says that our frame were made in secret in the LOWEST PART OF THE EARTH. What dies this refer to? What is the real meaning?
Note the comments of Ellicott about Ps 139:15 -
In the lowest parts of the earth.—This figurative allusion to the womb is intended no doubt to heighten the feeling of mystery attaching to birth. There may also be a covert allusion to the creation from dust as Ecclesiasticus 40:1, “From the day that they go out of their mother’s womb, till the day that they return to the mother of all things.” This allusion falls in with the view which meets us in other parts of the Old Testament, that the creation of Adam is repeated at every birth (Job 33:6, and see above, Psalm 139:13).
Others, since the expression “lowest places of the earth” is used of the unseen world (Psalm 63:9; comp. Psalm 86:13), see here a confirmation of the view that the state before birth and after death are in this poem regarded as the dark void of night, with all the recesses of which, however, God is acquainted. (Comp. the expressions “Womb of Sheôl,” “Belly of hell,” Jonah 2:2; Ecclesiasticus 51:5.)
Benson is similar:
In the womb; termed, in the next clause, in the lowest parts of the earth, in a place as remote from human eyes as the lowest parts of the earth are. He seems to allude to plants and flowers, the roots and first rudiments of which are formed under ground. And curiously wrought — Exquisitely composed of bones, muscles, sinews, arteries, veins, nerves, and other parts, all framed with such wonderful skill, that even heathen, upon the contemplation of the human body in all its parts, and observing how admirably they were formed for beauty and use, have broken forth into admiration and adoration of the Creator.
Thus, it appears that, the "depths of the earth" is a metaphor for the unseen world inside the womb.