If you are like me, you were never taught that the angels that rebelled against God, decided to come on earth and mate with women. When Satan rebelled, he took a third of Heaven with him. They were cast out by God and decided to make wives of the daughters of men. In Genesis chapter 6 it says:

"When human beings began to increase in number on the earth and daughters were born to them, ² the sons of God saw that the daughters of humans were beautiful, and they married any of them they chose. ³ Then the Lord said, “My Spirit will not contend with humans forever, for they are mortal; their days will be a hundred and twenty years.”

⁴ The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of humans and had children by them. They were the heroes of old, men of renown."

The book of Enoch goes into this in great detail. The book of Enoch was a part of the bible at one time. Enoch was so loved by God that he was one of the 2 confirmed people in the bible that never died. God literally took him up to heaven without death. 

In the book of Enoch (read the Ethiopian version only because the Hebrew and Slavic texts of the book are not part of the original biblical texts.) In 1Enoch Chapter 7 it says:

1. And all the others together with them took unto themselves wives, and each chose for himself one, and they began to go in unto them and to defile themselves with them, and they taught them charms and enchantments, and the cutting of roots, and made them acquainted with plants. 2. And they became pregnant, and they bare great giants, whose height was three thousand ells: 3. Who consumed all the acquisitions of men. And when men could no longer sustain them, 4. the giants turned against them and devoured mankind. 5. And they began to sin against birds, and beasts, and reptiles, and fish, and to devour one another's flesh, and drink the blood. 6. Then the earth laid accusation against the lawless ones.

So Basically, the fallen angels made hybrids with human women, and then the giants were making even worse hybrids with animals and humans. The fallen angels taught the women all the secrets of heaven which were not supposed to be disclosed to us. The giants started eating everyone and killing everything and each other. God saw that the earth was vile, and he had to restart humanity. He sent the flood.

We are now living in the days of Noah, same vile civilization part 2.