Jude 6 (NIV)
And the angels who did not keep their positions of authority but abandoned their own home--these he has kept in darkness, bound with everlasting chains for judgment on the great Day.
I am not given to much speculation when it comes to the Bible but I do believe what it says and there are some intriguing, mysterious things it says at times. This is not major doctrine here but I believe that there was a grave reason why God flooded the entire world in Noah's day. Certainly it was the wickedness of that day as is made clear in the passage starting with the following verse.
Genesis 6:5 (NIV)
The Lord saw how great man's wickedness on the earth had become, and that every inclination of the thoughts of his heart was only evil all the time.
However in Genesis 6:1-4 is a reference to an obscure group of people referred to as the sons of God and the Nephilim. It is commonly thought that the Nephilim were some sort of strange creatures, perhaps giants, but the word has the idea of the "fallen ones". In the Old Testament, the Hebrew phrase for "sons of God" always is a reference to angels or a direct creation of God (as opposed to us who are "sons of man").
Genesis 6:1-2,4 (NIV)
When men began to increase in number on the earth and daughters were born to them, the sons of God saw that the daughters of men were beautiful, and they married any of them they chose.
The Nephilim were on the earth in those days--and also afterward--when the sons of God went to the daughters of men and had children by them. They were the heroes of old, men of renown.
The allusion here is that somehow, fallen angels procreated with women, creating an awful half-breed of men. Now the Bible does not say that but I am sort of reading that into it. Like I said starting out, this is not a critical doctrine but fodder for imagination.
Peter refers to a group of angels who are already in a region of hell. I say region because the word for hell here is the Greek word Tartarus, which is supposedly an even lower region of hell. This word shows up in Greek mythology as the lowest and most terrible part of hell reserved for rebel superhumans (the only one I can think of is Hercules but he was not a rebel).
2 Peter 2:4 (NIV)
For if God did not spare angels when they sinned, but sent them to hell, putting them into gloomy dungeons to be held for judgment;
Perhaps these verses including my reading for today in Jude is a reference to the same event. Speculating further, could it be that in destroying all mankind except for Noah and his family, God was preserving a "pure" human lineage for Jesus Christ who would be born of man? What would have happened if he had not? The Devil would certainly have succeeded in foiling God's plan if he had been successful. Come to think of it, that was a key strategy all through the Old Testament from Pharoah killing the Jewish babies to Herod killing the babies around Bethlehem.
It would take a kinsman redeemer to save us. One who was our kin, a fellow human being, and one who was sinless and perfect, God himself. I am utterly astonished at the beauty and stark perfection of a plan that preserves God's justice yet allows him to be merciful too. Praise be to God!