The story goes, that when Humans were created, the Annunaki were attracted to the humans and started breeding with them. This wasn't acceptable with the "higher ups" and the Earth was flooded to destroy this experiment.
Very interesting story and it does explain a lot. How the Pyramids were built and all. The Great Flood and so on.
According to the Sumerians, the aliens finally left Earth so their "creations" could develop on their own. There is, however, the caveat that they would return sometime to see how well that experiment worked.
Isn't this what Christians believe as the 2nd coming of Christ? Sounds like modern day Religion is based on the Ancient Sumerian texts.
What's more believable to you? That God created the world in 7 days and the Earth is only 10,000 years old? Google "Annunaki" for more info.