A story that J R R Tolkien would have been proud of!
Book of Giants - one of the Dead Sea Scrolls
Enoch lived before the Flood, during a time when the world, in ancient imagination, was very different. Human beings lived much longer. Enoch's son Methuselah, for instance, attained the age of 969 years!
Another difference was that angels and humans interacted freely -- so freely, in fact, that some of the angels begot children with human females. This fact is neutrally reported in Genesis (6:1-4), but other stories view this episode as the source of the corruption that made the punishing flood necessary.
According to The Book of Enoch, the mingling of angel and human was actually the idea of Shernihaza, the leader of the evil angels, who lured 200 others to cohabit with women.
The offspring of these unnatural unions were giants 450 feet high! The wicked angels and the giants began to oppress the human population and to teach them to do evil. For this reason God determined to imprison the angels until the final judgment and to destroy the earth with a flood. Enoch's efforts to intercede with heaven for the fallen angels were unsuccessful (1 Enoch 6-16).
The Book of Giants retells part of this story and elaborates on the exploits of the giants, especially the two children of Shemihaza, Ohya and Hahya. Since no complete manuscript exists of Giants, its exact contents and their order remain a matter of guesswork. Most of the content of the present fragments concerns the giants' ominous dreams and Enoch's efforts to interpret them and to intercede with God on the giants' behalf. Unfortunately, little remains of the independent adventures of the giants, but it is likely that these tales were at least partially derived from ancient Near Eastern mythology. Thus the name of one of the giants is Gilgamesh, the Babylonian hero and subject of a great epic written in the third millennium B.C.E.