Just before the Flood, we read:
The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of man and they bore children to them. These were the mighty men who were of old, the men of renown. (Genesis 6:4)
Then, many years later:
And there we saw the Nephilim (the sons of Anak, who come from the Nephilim), and we seemed to ourselves like grasshoppers, and so we seemed to them.” (Numbers 13:33)
Most likely, the word nephilim means “giants” and both verses are simply saying that there were very tall groups of people both before and after the Flood, rather than indicating any ethnic connection between the pre-Flood and post-Flood nephilim.
It’s also worth knowing that it’s at least possible that the Flood wasn’t “universal” (i.e., it didn’t cover the whole earth), which would mean that some nephilim could have been unaffected by it (for more on this, see ‘Did the Flood cover the whole world?‘).
Perhaps it is possible, as well, that the wife of one of Noah’s three sons could be a converted descendant of the original Nephilim. Think of recessive vs. dominant genes, and you can see how possibly one of their children or grandchildren demonstrated the mutation that her DNA carried, causing ‘Nephilim’ to ‘appear’ in the earth again.
wrong interpretation.
“The Days of Noah”
The 24th century B.C.E. found badness overwhelming the earth. Moral standards were out of control. The historical record of Genesis 6:5 relates: “Consequently Jehovah saw that the badness of man was abundant in the earth and every inclination of the thoughts of his heart was only bad all the time.” In what way was it bad, and why?
Badness manifested itself in two bold ways. One, by its violence. Two, by its perverted sex. Notice how this is alluded to in Genesis 6:4: “The Nephilim proved to be in the earth in those days, and also after that, when the sons of the true God continued to have relations with the daughters of men and they bore sons to them, they were the mighty ones who were of old, the men of fame.”
The word “Nephilim” means “fellers,” or “fallers.” The Nephilim were bullies. They caused others to fall down violently. No doubt many followed their violent example or joined with them in plundering weak victims. The Nephilim were hybrid offspring from sex acts between materialized rebellious angels, who were at one time “sons of the true God,” and earthly women. Such sex between angels and humans was unnatural, perverted. (For additional information, please read 1 Peter 3:19, 20; Jude 6, 7.)
To what did this moral bankruptcy lead? “So Jehovah said: ‘I am going to wipe men whom I have created off the surface of the ground, from man to domestic animal, to moving animal and to flying creature of the heavens, because I do regret that I have made them.’ But Noah found favor in the eyes of Jehovah.” (Genesis 6:7, 8) Jehovah foreclosed on that morally degenerate system by means of the greatest cataclysm in all human history up to that time. Noah and his immediate family were the only humans to escape that deluge.
Why did Noah and his family find favor in the eyes of God? “Noah was a righteous man. He proved himself faultless among his contemporaries,” says Genesis 6:9. How did he do it? “Noah walked with the true God,” states the conclusion of that verse. Noah was courageous and distinguished himself from his immoral contemporaries by allowing Jehovah’s moral principles to set the boundaries for the way he and his family would walk through life. Noah flatly refused to let the world squeeze him into its mold of corrupted conduct.
See https://bibleq.net/answer/3550/ and https://bibleq.net/answer/1294/.