The World Below
(Now this, “He ascended”—what does it mean but that He also first descended into the lower parts of the earth? He who descended is also the One who ascended far above all the heavens, that He might fill all things.)(Ephesians 4:9-10) NKJV
After the crucifixion Jesus descended into the lower parts of the Earth. We’re to understand that he went to gather the souls of the Old Testament saints who’d been awaiting his coming.
While he was there he preached to the spirits in prison. We can understand this by incorporating what the Bible teaches us about the interior of the Earth into our model of its internal structure.
Something to be aware of is that the newer translations do something sneaky with punctuation to make it look as if Jesus descended to the Earth, the Earth’s surface being the lower parts of creation. This doesn’t change the fact that the universe has an absolute frame of reference, but it’s a capitulation with the popular science (SciPop) model of the internal structure of the Earth which doesn’t have hell or an underworld realm of the dead.
It’s very clear from seismological data that there’s a world below, it’s indicated by something called the Lehmann discontinuity. The issue of whether or not there is a chasm of open space inside the Earth between the lower mantle in hell, as described by Jesus, is resolved in our theory of gravitation.