Who descended and what are the lower parts of the Earth? Fortunately this isn't a mystery and we've already done the work needed to resolve both issues.
He Also Descended
Angels That Sinned
The dispensation of conscience, the time immediately after the fall of man when Adam and Eve were banished from the garden of Eden, we've also deduced was the first purification of heaven.
Everlasting Chains of Darkness
Hell wasn't created during the six days of creation. This is important to understand. Hell is a consequence of sin which began after the fall of man. God's curse upon the Earth is nuclear decay.
July 13th
Jesus descended into the lower parts of the Earth. We can resolve this as being a place in the lower mantle, we euphemistically call it Abraham's bosom, the same location that he described in Luke 16.