Daviddal, While the flood myth probably has some deep seated roots in the psyche of humans, like the hero myths, the garden of paradise, etc, it was likely based on a real event. Many ancient Near Eastern cultures had a flood event in their mythology, and we now know that the Black Sea was once landlocked and much smaller in size, with settlements around the shoreline. Something happened to cause the water from the Mediterranean to rush in, probably a huge earthquake, and the populated shoreline areas were completely flooded. The same research submarine that was used to locate the Titanic has photographed the ruins of early settlements/buildings on the bottom of the Black Sea. So this is probably where the flood story originated, at least in that part of the world. Over time it came to be mythologized, becoming part of religious traditions, etc, but it was probably based on a real event originally.
Portions of Greek mythology also had some basis in real events, as in the Trojan War. For centuries scholars thought it was all a myth, but now it's clear that there was in fact a war, or series of wars, to control trade through the Bosphorus, the waterway linking the Mediterranean and Black Sea. The rocky soil of Greece could never provide sufficient food for the population, so much of it had to be brought in by ship from the Black Sea region. So this was a very strategic spot, sort of an ancient Straits of Hormuz. 1500 years later, Constantine built his eastern Roman capital on the other end of the Bosphorus for similar strategic reasons.
Altantis is another myth that probably has some basis in fact. There are different theories, but it most likely represents the Minoan culture of Crete, which was the dominant maritime/trading power in the eastern Mediterranean for many centuries. Being centrally located, they were the main trading hub linking the Egyptians, the cultures of the ancient Near East, and the Mycenean Greeks. When the volcano on Thira exploded (modern Santorini), the Minoan culture was fatally weakened and then conquered by the Myceneans from mainland Greece. The idea of an Atlantis was first mentioned in the writings of Plato, though it's possible that he was merely using it as a literary device to represent a decadent culture ready for a fall. In any event, over the centuries, the idea of "Atlantis" took on a life of its own.