Jesus declared His triumph to imprisoned demons

1 Peter 3:18-20 (ESV)

^18  For Christ also suffered once for sins, 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,

^19  in which he went and proclaimed to the spirits in prison,

^20  because they formerly did not obey, when God’s patience waited in the days of Noah, while the ark was being prepared, in which a few, that is, eight persons, were brought safely through water.

Interpretation:  The imprisoned spirits, the same ones referred to in Jude 6, were demons who, in the days of Noah (see Gen 6.1-8) sought to corrupt the human race utterly, and so ruin God’s plan of bring a Messiah to crush satan’s head.  They did this by possessing the biggest (genetically large and strong, like Goliath was later), most aggressive, cruelest, most dominant men and raising up generations of dominant, dangerous people.  God dealt with them in two ways.  First by removing those demons from the earth and permanently incarcerating them (Jude 6) in the bottomless pit; and second by killing all those people in the flood. 

After He died physically Jesus, still spiritually alive, went to Tartarus where they were incarcerated and proclaimed ultimate victory over them, and over the sin and death in which they had trafficked.

This interpretation fits the facts provided in Genesis 6; though the rationale for why God (Jesus) considered it necessary to visit them in their prison and inform them of His ultimate victory is a mystery -- not explained here or elsewhere.