A.T. Robertson Commentary Ephesians 4:9

"(Now this, He ascended, what is it but that he also descended into the lower parts of the earth?" — Ephesians 4:9 (ASV)

Now this (το δε). Paul picks out the verb αναβας (second aorist active participle of αναβαινω, to go up), changes its form to ανεβη (second aorist indicative), and points the article (το) at it. Then he concludes that it implied a previous καταβας (coming down).

Into the lower parts of the earth (εις τα κατωτερα της γης). If the αναβας is the Ascension of Christ, then the καταβας would be the Descent (Incarnation) to earth and της γης would be the genitive of apposition. What follows in verse 10 argues for this view. Otherwise one must think of the death of Christ (the descent into Hades of Ac 2:31).