I was stunned to learn the geologic origin of the Biblical flood from Jeff Goodell’s book The Water Will Come and (Goodell’s source) Noah’s Flood by Walter Pitman and William Ryan.

The story of Noah’s Ark is widely accepted to originate with a similar story of a flood in the Epic of Gilgamesh, a poem from Mesopotamia that is about 4,000 years old, predating the Bible.

And the flood in the Epic of Gilgamesh, according to Pittman and Ryan, originated itself from an actual event that took place about 7,000 years ago as follows:

Variations in the angle of the earth’s tilt and the shape of the earth’s orbit around the sun cause ice ages and rising and falling sea levels. The last interglacial period was 120,000 years ago. Then, sea levels were 20 to 30 feet higher. The peak of the last ice age was 20,000 years ago, when sea levels were about 400 feet lower.
As the last ice age receded, the water from the melting ice caused sea levels to rise over the next 13,000 years. About year 7,000 years ago, the sea level had reached just about today’s level.

At the time, the Black Sea was a freshwater lake separated from the Mediterranean Sea by a land bridge. Small settlements existed around the Black Sea, fishing and farming for sustenance.

The Black Sea today (light blue) and in 5,600 BC (dark blue)

The land bridge separating the Black Sea and the Mediterranean Sea was under incredible pressure. Sea levels had been rising on one side of it for 13,000 years, until the water was 500 feet higher than the Black Sea. One day, the land bridge collapsed.
The magnitude of this event is hard to comprehend. The water created a massive path—what is today the Bosphorus strait. According to Pitman and Ryan, ten cubic miles of water rushed through each day. That’s enough water to cover Manhattan each day with water a half mile deep. The level of the lake rose six inches a day and moved upstream in the rivers by a half mile each day. As Pitman and Ryan state:

It’s hard to imagine the terror of those farmers, forced from their fields by an event they could not understand, a force of such incredible violence that it was as if the collected fury of all the gods was being hurled at them. They fled with family, the old and the young, carrying what they could, along with fragments of other languages, new ideas, and new technologies gathered from around the lake.

By two years later, the level of the lake had risen 330 feet and was about level with the sea. The people that had been living around the Black Sea had fled in every direction. Over time, these people and the story of what happened, made it to Mesopotamia, where the epic story they shared became legend and inspired stories in the Epic of Gilgamesh and later the Bible.