Bereshis 6:4-6

HaNefilim were on ha'aretz in those yamim; and also after that, when the bnei HaElohim came in unto the banot HaAdam, and they bore children to them, the same became gibborim which were of old, men of renown. And HASHEM saw that the wickedness of HaAdam was great in ha'aretz and that every yetzer (inclination) of the machshevot (thoughts) of his lev was only rah (evil) continually. And HASHEM relented that He had made HaAdam on ha'aretz, and He was grieved in His lev.