The evidence comes from both geologists and marine archaeologists. Geologists have found that around 5600BC water flooded from the Mediterranean through the Bosphorus at 200 times the rate of flow of the Niagara Falls (the huge waterfall lying at the borders of Canada and the USA), eventually forming a new sea. Marine archaeologists also found traces of ancient shorelines and manmade structures that date back to the same period lying 100 metres underwater off the sea’s coast in the area. Of course, the theory that the great flood actually created the Black Sea is a theory that is still to be proven.
Reference: (Black Sea: the Great Flood, TUNZA Vol 3 No.4 pg23)