The narrative of Genesis started with a story of Creation. It continued with a tale of an idyllic, child-like life for humanity; followed by a coming-of-age and loss-of-innocence story. Then the narrative continues with an introduction of of both worship and violence.
The thread of relations between mankind and their Creator isn't always obvious.
Adam and Eve have another son, naming him Seth. Seth later had a son, and "Then men began to call on the Name of the Lord".*
It's a change in the culture, or a change in circumstance. Or maybe a change in interaction with God, from one unstated form to another. Whatever the change was, it was significant enough to be mentioned in this story.
Later, during an enumeration of important men in the line from Adam to Noah, we meet another man named Enoch. He had a special interaction with God, such that he didn't die. He "walked with God; and was no more, for God took him."
Again, a very short statement about something that was distinct, but was not described in full.
Then things get to a shocking point. The evil nature of man has increased. The increase reached a point at which God Himself was grieved that He had created humankind.
But a select man, Noah, found grace in the eyes of God. And God gives him a plan of safety from the coming destruction.
Noah is warned about a cataclysmic flood, and instructed to build a large sea-going vessel.** He is instructed to bring his family, and all kinds of animals. Even in the words of warning, God promises protective covenants with Noah. Things are bad, but God promises that He is watching over the people He cares about.
Shortly after Noah enters his vessel, the rains come. It is described as a rainstorm that didn't stop for forty days, joined with some sort of underground fountains bursting forth. Mountains were covered, all living creatures were swept away. The vessel was floating on the water for months before it finds land. Nearly a year passes before plants are growing again.
The scale of this is mind-blowing. It is a restart of global proportions.
After Noah feels it safe to leave his vessel, he worships God. And receives a promise: never again will the ground be cursed because of man. Never again will a flood cover the entire earth. A regular rhythm of seed and harvest will happen.
Finally, God gives a direct order about interactions between humans. If any man murders another, then the murderer ought to be killed himself.
The story is astounding, even though it is full of things that seem strange and questionable to my mind. Was there really a global flood?*** How could so many animals survive a year in that vessel together? Noah was encouraged to take more clean animals than unclean animals. Were these the source of his offering to God at the end of the story?
Yet I am reminded that the point of this story is a developing relationship. God is trying to tell people how to get along with each other, and to remind them that He is the author of the rhythms of nature. He can, when He wishes, send cataclysmic judgement. Yet He has promised to never judge the entire Earth again, in the way that this story indicates happened at least once.
The sign of this promise is the sign of a rainbow. An arc of light in the clouds, visible to any man (under the right circumstances) is said to be a sign of God's promise.
* This sentence is followed by a sentence which most translations render something like, "This is the book of the generations of Adam."
That reminded me of something, so I did a little searching.
Genesis 2.4 "These are the generations of the heaven and earth..."
Genesis 5.1 "This is the book of the generations of Adam."
Genesis 6.9 "These are the generations of Noah."
Genesis 10.1 "Now these are the generations of the sons of Noah; Shem, Ham, and Japeth..."
Genesis 11.10 "These are the generations of Shem."
Genesis 11.27 "These are the generations of Terah."
Genesis 25.12 "These are the generations of Ishmael, Abraham's son..."
Genesis 25.19 "These are the generations of Isaac, Abraham's son..."
Genesis 36.1 "Now these are the generations of Esau, who is Edom..." (also repeated in v.9)
Genesis 37.2 "These are the generations of Jacob"
Some of the sections are short, some of them are long. Most of these statements follow the story of the person mentioned, but some precede the story.
Anyway, this structure is embedded into the old stories that were gathered into the book of Genesis. Some of the narrative irregularities of Genesis--like the two different version of the Creation of man and woman--may be explained if we assume that the stories were originally composed separately.
** It's often translated "ark", but it isn't the same Hebrew word for "ark" that was made by Moses. Noah's piece of nautical construction may have been termed a "life-saver". The Hebrew word only appears elsewhere when describing the wicker basket used to save baby Moses in the Nile. Since a baby-sized wicker basket and a gigantic wooden sea vessel have little else in common, the assumed commonality is "life-saver" or "salvation".
If the cubit given in the story is approximately 18 inches (~45 cm), then the vessel was 450 feet (~137 m) long, 150 feet (~45.7 m) wide, and 45 feet (~13.7 m) tall.
*** Narratives including a Flood-as-divine-judgement existed in many cultures of the time. So did narratives of demigods interbreeding with humans (compare the Nephilim of Genesis 6.1-4). So did tales of multi-century-long lifespans for patriarchs. (Of the 10 generations claimed from Adam to Noah, 6 of the men were said to live 900+ years. All but Enoch were claimed to live 700+ years.)
Is this a sign that God was able to use narratives that were of a type known to exist among those cultures, in that time? Or that people did, at one time, see such a Flood and live to such ages?