I went down to the bottoms of the mountains; the earth with her bars was about me for ever: yet hast thou brought up my life from corruption, O LORD my God.(Jonah 2:6) KJV
Jonah wakes up in the underworld realm of the dead, sheol. From his vantage point he can see across a chasm of open space between the lower mantle, stretching out above him, and the surface of hell down below.
He can see the pillars of the earth protruding like broken spokes downward from the ceiling of the vast cavern. The earth with her bars would be a tricky one to figure out if it weren’t for the fact that Jonah’s painted a picture of being in a cavern from which he can can see columns reaching down from the inner surface of the lower mantle to where they melt in the molten core of the earth – hell.
Here’s the clue that we need: Jonah previously mentioned crying out from the belly of hell מִבֶּ֧טֶן שְׁא֛וֹל – mibbeten sheol. Now he’s referring to the same place as the bottoms of the mountains לְקִצְבֵ֤י הָרִים֙ – qetseb har. The word qetseb means moorings, something to which a boat is attached by a rope, or the rope used to moor a boat. Mountains are part of the crust of the Earth, they can only go down as far as the bottom of the lower mantle. The moorings of the mountains are the pillars of the earth. This means that the belly of hell is in the mantle, and that the pillars of the earth connect the mantle to the core.