During the days of Noah there was a group of Angels which had sexual relations with human beings and corrupted the human DNA with which man was created. The result of this led to giants being born in the earth, the human race was completely corrupted except for Noah and his house hold. These Angels were set as watchers of the human race in the earth, they had power to transform themselves into humans (Gen 6:4). This is why God had to destroy that generation by water, for had they remained, we would have been born half man and half demon, and the blood of Jesus Christ would not have been able to redeem us. All those Angels who committed this sin have been bound in the second heaven and they await the great judgment day. Jesus said before his coming what happened in the days of Noah will happen again (Matthew 24:37 But as the days of Noe were, So shall also the coming of the Son of man be). Have you seen how Hollywood has been coming up with movies of creatures coming from space to live in the earth with humans, the Ufos, Aliens we see on the movies is just a programming they are trying to put in us so that when those creatures come, we can accept them. In these last days, the wickedness in man has reached its highest, the so called ufos will be the fallen angels which will come to our world, again they will marry and have children with human beings. These Angels will come with the permission of human beings, God will not be able to stop this because man has the authority to decide what he allows into the earth. There are two groups of fallen Angels, there are those who fell with Lucifer and those who fell in the days of Noah. The ones which will be coming to earth will be those who fell with Lucifer, again God will wipe off humanity but this time with fire.