From The Book of Enoch

Found in the Qumran caves of the Dead Sea Scrolls (Qumran Cave 4) and presumed to be written before 170 BC, chapters 1-36 of 1 Enoch are also referred to as 'The Book of the Watchers.' These passages are also known to incorporate portions of the 'Book of Noah.'The theme of the Watchers is final judgment and incorporates the expository background of one of the most spectacularly glossed-over, overlooked, and misunderstood Scriptures in the Bible - Genesis 6:1-4. The "sons of God" referred to in Genesis 6:2 are explained in detail in The Book of the Watchers. These Watchers were supposed to be "watching' the earth as part of their heavenly ministry, but instead they transgressed and became fallen angels. How did they become fallen angels? They saw that women on earth were pleasant to look at and schemed/colluded to somehow incarnate themselves (literally celestial or, dare we say, 'extraterrestrial' beings manifesting in some kind of physical form) in order to impregnate women and corrupt the bloodline of humans.