What was Jesus Doing in Hell?

Short answer: Reaching out to the people who needed him most.

Some of the standard texts connected to this doctrine are:

Job 38:17 (NIV)
17 Have the gates of death been shown to you?
Have you seen the gates of the shadow of death?

Psalm 16:9-10 (NIV)
9 Therefore my heart is glad and my tongue rejoices;
my body also will rest secure,
10 because you will not abandon me to the grave…

Psalm 68:20 (NIV)
20 Our God is a God who saves;
from the Sovereign LORD comes escape from death.

Ephesians 4:9-10 (NIV)
9What does "he ascended" mean except that he also descended to the lower, earthly regions? 10He who descended is the very one who ascended higher than all the heavens, in order to fill the whole universe.

1 Peter 3:18-22 (NIV)
[Jesus] was put to death in the body but made alive by the Spirit, 19through whom also he went and preached to the spirits in prison 20who disobeyed long ago when God waited patiently in the days of Noah while the ark was being built.

1 Peter 4:5-6 (NIV)
5But they will have to give account to him who is ready to judge the living and the dead. 6For this is the reason the gospel was preached even to those who are now dead, so that they might be judged according to men in regard to the body, but live according to God in regard to the spirit.

The Apostles' Creed
This is a creed (an encapsulation of beliefs), which developed between the second and the ninth centuries after Christ, which many people in traditional liturgical churches are familiar with. Nineteenth century Methodists removed the phrase which mentions the descent into hell from their recitation of this creed, claiming it was unbiblical.

I believe in God, the Father almighty,
creator of heaven and earth.

I believe in Jesus Christ, God's only Son, our Lord,
who was conceived by the Holy Spirit,
born of the Virgin Mary,
suffered under Pontius Pilate,
was crucified, died, and was buried;
he descended to the dead.
On the third day he rose again;
he ascended into heaven,
he is seated at the right hand of the Father,
and he will come again to judge the living and the dead.

I believe in the Holy Spirit,
the holy catholic church,
the communion of saints,
the forgiveness of sins,
the resurrection of the body,
and the life everlasting. Amen.

The Harrowing of Hell
Sometimes Jesus' descent to hell is called the "harrowing of hell." In agriculture, a harrow is a tool for breaking up dirt. Harrowing is done in order to kill weeds and prepare a site to be seeded.

It seems inconceivable to me, having seen the relentless incursions of Jesus into the lives of the broken, that there might be some excluded from Jesus' fearless, fearsome love. I believe nothing – no darkness, no pain, no bereftness – is immune from his gracious presence. There is no place you can go, no darkness so deep that you are beyond the reach of God. He is willing to go to hell in order to bring home his harvest.