Ephesians 4:9-10. “(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.)”
It seems there are two basic position on these two verses. The first is that the meaning is that Jesus descended from heaven to earth, and then went back again when His ministy on earth was completed.
The second is that the “lower parts” of the earth refers to Hades. The belief here is that on His death, Jesus descended, as the spirits of all Old Testament believers did, into Hades; there, He freed the captives who had been held there and took them to heaven with Him. Proof texts for this position include Eph. 4:8-10; Psalm 16:10;68:18; Matt. 12:40; Heb. 2:14-15.
The following verses are proof texts that hell exists in the deepest parts of the earth, not just the grave: Matt. 12:40; Psalm 16:10; 63:9; Job 11:8; Deut. 32:22; Is.14:9; 66:22-24; Ps. 9:18; I15:24; Exek. 31:14-19; 32:18-24.
Hell is contrasted with the highest heaven as being the lowest and deepest part of the earth. It could not possibly refer to the grave on the surface of the earth.
In my own memory, I have been taught this second position. It explains what Jesus was doing during the time between His death and resurrection. However, very few of the commentaries I consulted even addressed these two verses, which I found interesting. The ones that did were pretty evenly divided between the first position and the second.
I’m going to leave it to you to study for yourselves, and feel free to tell me what you discover.