When men began to increase in number on the earth
and daughters were born to them, the sons of God
saw that the daughters of men were beautiful, and they
married any of them they chose. Then the LORD said, "My
Spirit will not contend with man forever, for he is mortal;
his days will be a hundred and twenty years."

The Nephilim were on the earth in those days-- and
also afterward--when the sons of God went to the
daughters of men and had children by them. They
were the heroes of old, men of renown.

--Genesis 6:1-4