Genesis 6:4 - There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.

Genesis 6:1-7 are some of the most enigmatic verses in the OT. What are these verses referring to? Was there actually giants living on earth before the Great Flood (anti-diluvian period)? There are early Jewish writings and other ancient text which expound upon these verses. Josephus (The Book of Antiquities, Book I, Chpt. 3) implies they are related to the Greek Gods:
Many angels of God now consorted with women, and begot sons on them that were overbearing and disdainful of every virtue. Such confidence had have in their strength. In fact, the deeds that our tradition ascribes to them were called the audacious exploits told by the Greeks of the Giants. But Noah urged them to adopt a better frame of mind and amend their ways.
For many angels of God accompanied with women, and begat sons that proved unjust, and despisers of all that was good, on account of the confidence they had in their own strength; for the tradition is, that these men did what resembled the acts of those whom the Grecians call giants. But Noah was very uneasy at what they did; and being displeased at their conduct, persuaded them to change their dispositions and their acts for the better,,, [http://www.earlyjewishwritings.com/text/josephus/ant1.html]
Altho not specifically mentioned by Austin, I found this passage in Philo of Alexandria to follow the general tone being set:
II. (6) "And when the angels of God saw the daughters of men that they were beautiful, they took unto themselves wives of all of them whom they Chose."{2}{#ge 6:2.} Those beings, whom other philosophers call demons, Moses usually calls angels; and they are souls hovering in the air. (7) And let no one suppose, that what is here stated is a fable, for it is necessarily true that the universe must be filled with living things in all its parts, since every one of its primary and elementary portions contains its appropriate animals and such as are consistent with its nature; --the earth containing terrestrial animals, the sea and the rivers containing aquatic animals, and the fire such as are born in the fire (but it is said, that such as these last are found chiefly in Macedonia), and the heaven containing the stars: (8) for these also are entire souls pervading the universe, being unadulterated and divine, inasmuch as they move in a circle, which is the kind of motion most akin to the mind, for every one of them is the parent mind. It is therefore necessary that the air also should be full of living beings. And these beings are invisible to us, inasmuch as the air itself is not visible to mortal sight. (9) But it does not follow, because our sight is incapable of perceiving the forms of souls, that for that reason there are no souls in the air; but it follows of necessity that they must be comprehended by the mind, in order that like may be contemplated by like. [Philo, On the Giants II: 6-9 - http://www.earlychristianwritings.com/yonge/book9.html]
The flood of the Bible and the Greek flood myth are remarkably similar. Is it possible that the Greek myths are not myths? Was Zeus a Nephilim? Nephilim, is the Hebrew words for giants. Was his father (Cronus) a fallen angel called “sons of God” in Genesis 6.

The ancient Greeks believed in a flood that destroyed all mankind, similar to the account described in the Bible. In the Greek myth, the flood took place during the age of Deucalion.

Deucalion and his wife, Pyrrah, were warned in advance of the coming deluge and told to construct a chest and fill it with provisions. The source of the coming deluge was the anger of Zeus towards mankind. Zeus had become disgusted at the degeneration of humans, and in particular, the practice of human sacrifice. Zeus assembled the Gods (Psalms 82 - Divine Council) and decided to wipe out mankind by water. Deucalion and Pyrrah survived the flood by floating in the chest they built.

There are a great number of differences between the Greek myth and the account of the Flood described in the Bible. However, both accounts maintain that the world was destroyed as the result of mankind's sin, and the human race was saved as a result of a few individuals leading a righteous life.

This same story is world wide; every culture has a flood story. (Can one surmise that there may be truth in it's origin? Which story came first? What about Epic of Gilgamesh, which predates all?) Many of these story refer to the Giants before the Flood just as the Greeks refer to the Gods - the Titans and the Olympians.

Many scholars are making the association of the Greek Gods and the Nephilim and the fallen angels of the Genesis account. The Titans being the fallen angels or as the Book of Enoch calls them, Watchers; and the Olympians being the Nephilim.

The discussion of giants on earth goes much further and more deeper than just this comparison:
Chuck Missler clip-

If you study classic Greek mythology you run into the Greek Titans -
  • partly terrestrial, partly celestial
  • rebelled against their father Uranus
  • after prolonged contest were defeated by Zeus and condemned into Tartarus

You see a parallel brewing here, I’m going to suggest to you that the legends of the ancient Greeks, embody, the truth of what really happened in the past; that there were these strange creatures ,,, hybrids that the Greeks called Titans.

,,,Atlas and Hercules, from Greek mythology, were what would be called in the Hebrew, Nephilim (offspring from the inter-marriage of a God and a earthly woman.)

These legends are seen in other cultures: Sumer, Assyrian, Inca, Maya, Epic of Gilgamesh, Persian, Greek, India, Bolivia, South Sea Islands, American Indian

 Every one of these cultures have legends of the star people, these Gods or demi-Gods that came and cohabitated with women and produced hybrids.