The Nephilim in the Old Testament were the offspring of the “sons of God” and the “daughters of men” before the Deluge, according to Genesis 6:1-4. Many early translations of the OT, as well as the King James version, preferred to translate the original Hebrew word as ‘giants’.
The World Heritage Encyclopedia offers a range of theories regarding the etymology of the word(a). Wikipedia has further background information on the subject(c).
In more recent times it was the work of the late Zechariah Sitchin that revived interest in the Nephilim, the Anunnaki and the existence of extraterrestrial visitors in ancient times. Unlike the more than dubious claims of Erich von Däniken, Sitchin’s ideas appeared to have a more reliable scientific foundation. However, this foundation was based Sitchin’s own interpretation of Sumerian texts, which has been heavily criticised(b).
In 2019, Ryan Pitterson, tried to revive the idea of a connection between Atlantis and the biblical Nephilim, but failed miserably. He is the author of Judgement of the Nephilim , which he claims to be “the first comprehensive biblical study of the nephilim.” Promotional interviews for the book gave him his fifteen minutes of fame, which is more than this book deserves.