Earth apparently begins to grow warmer about 12,000 years ago, causing the melting of the vast sheets of ice that most scientists believe sprawled over the Northern Hemisphere.

Oceans and seas grow deeper as a result, causing many fresh-water bodies, such as the Irish Sea, to fill with oceanic salt water.

One enormous lake in western Asia, separated from the salty Mediterranean Sea by a high plateau, remains "sweet." In the late 1990s, Columbia University geologists William Ryan and Walter Pitman wonder what could explain the preponderance of flood legends.

They theorize that sometime around 5000 BCE, the Mediterranean Sea swells and seawater surges northward, slicing through what is now Turkey and cutting the Bosporus.

Funneled through the narrow Bosporus, a wall of seawater hits the low-lying freshwater lake with 200 times the force of Niagara Falls.

Each day the lake level rises about six inches (15 centimeters), flooding coastal farms, inundating whatever centers of civilization exist, and forms what comes to be called the Black Sea.

According to this theory, survivors' tales of this great deluge are passed down through the generations to form the basis for the Mesopotamian saga of Gilgamesh and, eventually, the Biblical story of Noah.

A 1999 expedition headed by maritime explorer Robert Ballard reveals an ancient shoreline.

Also found are shells from freshwater and saltwater mollusk species.

Their radiocarbon dates support the theory of a freshwater lake inundated by the Black Sea some 7,000 years ago.

A 1998 expedition, says Ballard, reports "a series of features that appear to be man-made structures." To see what's happening in the world of higher sea levels, let's explore the epoch from 6407 to 4680, a period of emergent New Stone Age cultures, in which humans first live in settled villages, domesticate and breed animals, cultivate grain crops, and practice pottery, weaving, and flint-mining.