We all know the story of the flood, but sometimes we overlook the little prequel.
Ready? Go. The "sons of God" think that human females are hot tickets. We've been suspecting that God's not alone (remember the plurals of 1:26 and 3:22), and now other divine beings play an active role in the story.
Take a time out to join us in recognizing that some of these archaic strands within the Hebrew Bible reveal a degree of—big word alert!—"polytheism," that is, the acceptance of several deities, opposed to "monotheism" or the affirmation that God is singular.
So these sons of God start marrying these ladies. Greek myth is full of gods like Zeus who fall in love with mortal women, so this isn't out of the ordinary.
In response, God decides to limit the lifespan of mortals to a mere 120 years. Here again the Hebrew is back to God's nickname YHWH (the Lord).
The gods make it with their mortal brides, and they give birth to the "Nephilim" (6:4 NRSV), who are part-divine beings who walked the earth in pre-flood times. Nephilim is more or less a straight transliteration of the Hebrew word, which literally means "fallen" (without the connotations of the Christian concept of "the fall").
In a vestige of ancient Hebrew myth, the narrator simply says of these fellows that they "were the heroes that were of old, warriors of renown" (6:4 NRSV).