Angels, Mt.Hermon, Bashan, Og, Zuzim, Rephaim, Nephilim, Anakim
Its believed a second group of watchers fell. And the reason they're not mentioned in the Bible more is because it didn't line up with the story line of the Bible (which is to tell the story of Christ's lineage, proving it pure).
And to all you people who believe the "sons of God" are from the line of Seth, you're deluding yourself. Its naive to give Seth that much credit. Also, the tribes of the giants, notice they all end in "-im". They're all giants, just different tribes. Read Jude, Genesis, Numbers, Joshua, 1 & 2 Samuel, 1 & 2 Peter.
Basically, the whole Bible, because they're mentioned every now and then. The whole point of Satan corrupting these angels, tempting them with women and the thought of offspring in their own image, was to cut off any chance of Christ coming to earth to save us.
Think about it. Noah was perfect in his generation. It doesn't mean goody-goody perfect Christian. It means his lineage wasn't deformed by angels' blood. That's why it profited Satan to have the Watchers fall to earth. They ravaged the earth and killed the innocent, until all that was left were the giants with their children, and the humans that killed with them.
Then Noah and his family were the last human family on earth that believed in God. Read The book of Enoch. And what about Arba, Anak (Arba's son), Goliath and his three brothers, Og. They were nine feet tall or more! Og, king of Bashan had an iron bed that was thirteen feet long! And that's mentioned in the Bible!
You simply cannot say that angels didn't fall to earth. There's too much evidence in the Bible.
Also, there are no female angels. God made angels, then he made the earth, then he made man. Woman came from man. So, all angels are male. Their kids, on the other hand, could've been either.