"descended into the lower parts of the earth" (4:9). What does that mean?

The letter asks its own question about the line's words, "Now this, “He ascended”, what is it but that he also first descended into the lower parts of the earth?" (4:9), and goes on, "He who descended is the one who also ascended far above all the heavens, that he might fill all things." (4:10). It says no more about the lower parts of the earth than that, and these notes don't either. Christians read the descent in more than one way; these notes choose no reading and set no other verse beside it. Favor's Psalm 68, where one of the two lines that land on these verses starts, asks a question of its own (see Asked elsewhere in Favor).