A word frequently translated as the grave is the Hebrew שְׁא֣וֹל – sheol, but sheol is far more than just the grave. The grave is a portal into the underworld realm of the dead.
Once you enter the great gulf nothing can stop your fall into the core. The pit is everything below the mantle. If the sides of the pit is the lower mantle, then the pit includes the great gulf.
The levels of hell aren't what you think. However, they're what we would expect. The Hebrews had a well developed idea of life after death. For them physical death, the grave, was a portal into a complex underworld realm: sheol.
Seismological data shows different zones of density within the Earth which match the description of Earth's interior given us by Jesus in Luke 16:19-31, the parable of the rich man and Lazarus.
The deep and the great deep are both translated from the same Hebrew word, tehom. The difference signifies a transition from the deep where Earth was created to when the great deep is in the interior of the Earth.
We've found that the previous theory about the internal structure of Earth was induced, but no body knew any better so it was OK. We were in a state of blissful ignorance, assuming that ignorance was knowledge.
Jesus describes the internal structure of the Earth in the parable of the rich man and Lazarus. We use this passage to understand that sheol includes a chamber in the lower mantle which overlooks hell.
Continuing the theme of weird things that people say on social media there's another question which comes up from time to time which pertains to Jesus' teaching on hell.
"The earth with her bars" is how Jonah described the pillars of the earth when he was in the belly of hell, possibly in the place which we refer to as Abraham's bosom. Jesus also went here after the crucifixion.
"The pillars of the earth" are incorporated into a 3D model of a spherical Earth in which they're part of Earth's internal structure. They don't require us to believe that the Earth is flat.