It all began in 1971 as a game of speculation - a bit of light relief from the rigours of tectonic and oceanographic investigation in the Mediterranean basin. American oceanographer Bill Ryan was on the scientific team which showed that the Mediterranean had been a vast desert basin until, 5 million years ago, the world's oceans rose and burst through the Straits of Gibraltar to create the Mediterranean Sea.

Walter Pitman was helping to found the new science of plate tectonics. Ryan and Pitman's British associate John Dewey (now Professor of Earth Sciences at Oxford) put up an ingenious idea - could a similar cataclysmic flooding of a massive basin account for the Biblical Flood - a catastrophe of such enormity that it would remain in human memory down the ages? If so, where might it have taken place? The idea never went away and 20 years later, in 1991, Ryan and Pitman began their search. This year they will announce their findings to the scientific world. Richard Curson Smith's film takes Pitman and Ryan back to the Black Sea, where they now believe that the Flood occurred in 5,600 BC.

Filmed on location in Turkey, Bulgaria, Russia and the United States, HORIZON tells the story of the discovery through the eyes and experiences of the two scientists. Like Laurel and Hardy, they swing from disappointment to despair, to euphoria, as they pursue their dramatic idea, working with geological evidence from the Turkish navy, drilling into the bed of the Black Sea alongside a team of Russian scientists (who were tracking fall-out from Chernobyl), building in all the data available from experts in climate and tree-ring dating, and finally testing their findings on their colleagues in the Lamont Doherty Earth Observatory at Columbia University, and at the Woods Hole Oceanographic Institute.

Bill Ryan and Walter Pitman may have a romantic streak in their natures, but there's nothing eccentric about their ideas. They are, says Professor Dewey, "Two of the finest American earth scientists working in the field". They have gathered a hoard of evidence to demonstrate that the Black Sea had fallen 120 metres below the level of the worlds oceans as a result of a sudden freezing of Northern Europe and Asia which dramatically reduced the flow of the great rivers which fed it.

The Black Sea became a vast fresh-water lake. The Earth warmed, the oceans rose again and smashed through the Bosphorus Straits with the force of a hundred Niagara Falls, filling the Black Sea basin to its present level with the salt water of the oceans. The implications are enormous. Who lived by the Black Sea before the deluge? What may it tell us about the spread of Neolithic farming, culture and technology into Europe and beyond? Did this extraordinary event become the stuff of ancient storytelling, so that Noah and his Ark became a symbol for real people who were driven from their lands by a real flood?