According to the Book of Enoch, the Nephilim were the reason why God had to send the great flood, because these creatures had caused so much death and destruction that life on earth was nearly wiped out.
God sent the flood to wipe out everything, including the Nephilim. We know from this account that nothing survived the flood except for Noah and his family.
However, the Nephilim are seen many years later here in Numbers 13 by the spies of Moses:
'And they spread among the Israelites a bad report about the land they had explored. They said, "The land we explored devours those living in it". All the people we saw there are of great size.'
"We saw the Nephilim there, the sons of Anak, who come of the Nephilim. We seemed like grasshoppers in our own eyes, and we looked the same to them."
(Numbers 13:32-33)
Anak was believed to be an obscure figure that is mentioned in the Bible. He was the forefather of the Anakim, a Rephaim tribe, Goliath was part of this tribe.
These Nephilim, which the spies note were the descendants of Anak did somehow survive the flood or were somehow born after the flood.
So what is the reason for the reappearance of the Nephilims after the flood?
One theory is that the same event transpired later in history, as angelic beings again bred with women and produced more Nephilim.
Another is that Nephilim genes were passed down through Noah’s daughters in law.