(Now that he ascended, what is it but that he also descended first into the lower parts of the earth? He that descended is the same also that ascended up far above all heavens, that he might fill all things.)(Ephesians 4:9-10) KJV
The lower/nether parts of the earth is the lower mantle which we have identified as the foundations of the world/mountains/hills and the belly of hell. The pit is the molten core of the earth.