As I have started reading many religious text like the Bible, Gilgamesh and Ziusudra, I noticed all of them have stories of massive floods. Is this just a coincidence?
Well, it's not really a historical answer, per se, but Charles Pelligrino --among others-- has suggested that at least where those traditions connected with the middle east are concerned, the flood stories may well be Mesopotamian in origin since both the Tigris and Euphrates are known to have flooded and changed course numerous times during the last 10k years.
Another even more conjectural idea is that flood myths are related to an ancestral memory of the end of the last ice age when sea-levels across the globe rose precipitously, drowning many low lying coastal lands. I'm a bit skeptical of both ideas, but there they are.
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Weren't traces of settlements discovered under the Black Sea? When did the Mediterranean Sea spill into the area of the Black Sea?