Most people are familiar with the story of the Great Flood, or Deluge, in the Bible. What is less well-known is that there actually were deluges in the past, prehistoric deluges, mostly as a consequence of rising sea levels due to the melting of the ice at the end of the last Ice Age, for an extended period between 18,000 and 8,000 years ago. These may have inspired flood myths.
During the last Ice Age, northern Eurasia and North America were covered in thick glaciers, rendered uninhabitable. As a sort of tradeoff, large areas of land now underwater were once dry, because so much of the world's water was locked up in ice caps. This includes Doggerland, the area of the North Sea between the UK and the Netherlands; Beringia, which early man crossed to gain access to the Americas; Sundaland, a tropical region in present-day Indonesia, and many others.
As the ice melted, sea levels rose, and inundated the land. Usually this process was too slow to notice, occurring over hundreds or thousands of years, but sometimes, it was fast enough to notice in a human lifetime, and occasionally, it is thought to have occurred catastrophically, as prehistoric deluges.
One of the most frequently discussed deluges is the hypothetical flooding of the Black Sea region, which, if it happened, occurred about 5,600 years ago. The evidence comes in the form of large sills, or furrows, which would have been caused in a catastrophic overflow. Around the time of the last Ice Age, the Black Sea would have been disconnected from the Mediterranean. As the Mediterranean rose from water from melting ice caps, it would have risen above the crucial level to let water through the Bosporous and into the Black Sea, increasing its depth by as much as 300 ft (100 m). This prehistoric deluge would have flooded 60,000 square miles of land.
Other prehistoric deluges are hypothesized to have occurred in the Caspian Sea, overflowing into the Black Sea, the Carpentaria plain between Australia and New Guinea, the Aegean basin, and Doggerland. Some prehistoric deluges are thought to have been caused by the rupturing of ice dams, such as the Missoula floods in present-day Washington state; Lake Agassiz, located near the present-day Great Lakes but greater than all their volume combined; Lake Bonneville, located in the present-day Great Basin; and Lake Objibway, just north of the present-day Great Lakes. Prehistoric deluges, especially caused by the coming and going of glacial periods and glacial lakes, appear to be quite common on geologic timescales.