The Passage Itself
Genesis 6:1-4
Four verses describe sons of God taking daughters of men, a shortened lifespan verdict, and Nephilim on the earth — then move straight into the Flood narrative.
Primary text
Genesis 6:1-4 states that when men multiplied, the sons of God saw the daughters of men and took wives of all they chose; that the LORD limited man's days; that the Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men and they bore children to them — the mighty men who were of old, men of renown.
What the text does not say
It does not define 'sons of God'. It does not say the Nephilim were the offspring — grammatically they are on the earth when the unions happen. It does not say they were unusually tall. It gives no names, no origin story, and no explanation of 'and also afterward'.
Word note
Nephilim (נְפִילִים) is usually connected to the root n-p-l, 'to fall' — hence 'fallen ones'. Some read it instead as 'those who cause others to fall'. The ancient Greek Septuagint translated it gigantes, 'giants', which is why English tradition inherited the giant framing.
Open question
Why does a book this economical with words include the passage at all, and then refuse to explain it? Every interpretation below is an attempt to answer that.