Giants = Giants

Were the Nephilim (giants) invented by ancient Israelites, or are there other reliable sources that back up the Biblical claims?
Genesis 6:1-5 (KJV) “And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years. 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 bore children to them, the same became mighty men which were of old, men of renown. And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.”

The Church Fathers believed in the reality of giants that came about as a result of intermingling between the divine sons of God and the human daughters of men. Here is a sample of their writings from the first centuries of the church:
“The angels transgressed [their] appointment and were captivated by love of women.” - Justin Martyr
“Just as with men..., so is it among the angels. Some... outraged both the constitution of their nature and the government entrusted to them... (you know that we say nothing without witnesses, but state the things which have been declared by the prophets); these fell into impure love of virgins, and were subjugated by the flesh, and became negligent and wicked in the management of the things entrusted to [them]. Of these lovers of virgins, therefore, were begotten those who are called giants.” - Athenagoras
“Angels were united with the daughters of the race of mankind; and they bore to them sons who for their exceeding greatness were called giants... of the world of the second race.” - Irenaeus
“The author ... asserts that those [Nephilim] whom he defines with such a name because of the extraordinary size of their body were generated by angels and women.” - Ambrose

Even non-Christian sources gave testimony to the ancient giants:
“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. […] There were till then left the race of giants, who had bodies so large, and countenances so entirely different from other men, that they were surprising to the sight, and terrible to the hearing. The bones of these men are still shown to this very day, unlike to any credible relations of other men.” - Josephus

There were many other statements made by the church fathers in their writings of the first centuries of Christianity. You can find more of them in "Answers to Giant Questions."
But these attestations only give us very late textual evidence. How can we be sure they weren't just allegories or "colourful storytelling"? What we need is something closer to the historical context of the giants.
What other evidence do we have of the reality of the Nephilim?
Perhaps we might consider something from outside of the Judeo-Christian worldview and a few centuries closer to the days of the giants. Pausanias, in his 2nd century BC work, "Description of Greece," wrote that a "son of Anax", named Asterios, was buried in ancient time on the island then named after him, and that his corpse measured not less than ten cubits (fifteen feet) in length. Perhaps now we understand more clearly why the Israelites were terrified at the first mention of the "sons of Anak" as recorded in the book of Numbers - before their incredible height was even mentioned by the spies.
What this means is that the Greek translation of the Hebrew Bible (the Septuagint, or LXX, translated in the 3rd - 2nd centuries BC) shares a common vocabulary with extra-biblical sources of the same era, with shared definitions. In other words, today's Bible (if you have one that actually translates the words instead of employing transliteration) preserves the same meaning that the Greeks of that day understood. When they said "giants," they meant it the same way we moderns understand it - Very Big People.
That translation in the LXX achieves more than just to inform us that the Anakim were tall by association with the Rephaim (translated "gigantes" in the Greek). Because the same Greek is used to translate the earlier 6th century BC Aramaic "Naphilin" (plural form of Naphila, which means "giant") which was preserved in the Hebrew Bible as "Nephilim." And that explains to us why a term that most take to mean "fallen ones" really cannot mean anything other than "giants" - as we understand the word today.

- T.J. Steadman