For Christ also died for sins once for all, the righteous for the unrighteous, that he might bring us to God, being put to death in the flesh but made alive in the spirit; in which he went and preached to the spirits in prison, who formerly did not obey, when God's patience waited in the days of Noah....
It's very explicit that he encountered spirits of people from the Old Testament. But just in case you thought Peter wasn't talking about dead people:
For this is why the gospel was preached even to the dead, that though judged in the flesh like men, they might live in the spirit like God.
Now I know that the apostles preached the Descent, but I still don't know how exactly they knew about it. A likely conclusion is that it's one of the things Christ told them between his Resurrection and Ascension. It's very exciting to think about all the things he told them during that time. How awestruck they must have been to hear him speak about this event in particular. That would really give me hope that the Resurrection wasn't just for the God-man, but for me as well (just in case the graves opening up in Jerusalem and dead people walking around wasn't enough.)