There is a series of non-fiction books that deal exclusively with who these mysterious visitors to earth were. The author is Zecharia Sitchin, and this series is known as “The Earth Chronicles.” Sitchin proves that most of the questionable antics in the Old Testament are in fact edited versions of Babylonian and Sumerian stories.
Basically what Sitchin posits, is this:
the Nephilim were a group of astronauts from another planet, this planet was called Nibiru by the Sumerians. They came to earth several hundred thousand years ago. They established the first human civilization by laying out their settlements in a grid, establishing their base in Sumer. These people were also called “Annunaki”. The Annunaki took primitive humans and spliced Annunaki DNA into them, to create a “worker.” Later, these workers were perfected so that they too could procreate. After many thousands of years, the Annunaki’s children and the new homo sapiens began interbreeding, as the quotation in the Bible mentions “…sons of the gods taking the daughters of men as wives…” etc.
The Annunaki home planet was again nearing earth, and the Annunaki saw that the gravitational effects would cause the ice sheet at the poles to slip, causing tidal waves and an eventual deluge. The Annunaki left earth and waited out the Flood in their orbiting spacecraft, while most of humanity perished. The “seed” of humanity was preserved, as one of the sons of the ruler of the Annunaki (Enki, the benefactor of mankind) instructed his servant Utnapishtim to make a submersible boat to survive the Flood. This person is also called Noah, but the names mean the same thing.
The Annunaki founded the Indus River Valley civilization, a faction moved into Egypt and became known as Ptah, Ra and others, and Ishtar, Innanna and many other “gods” were really just the children of the original Annunaki and their earth-partners. And so on.
The books are incredibly fascinating, and give a wealth of history about the foundations of civilization, even if one rejects the central premise of beings from another planet ruling earth. They are a great read, wonderfully well-written, and are vastly entertaining.