The ocean levels have changed many times due to vulcanoes and meteors throwing up white particles causing permafrost. Then people build cities lower, climate gets warm again, ice melts fast during monsoon season and they are flooded. Greece has many ancient houses below ocean levels to show that these kind of events actually happened locally, which to their limited knowledge was the entire world.There was a great flood on Earth. This flood submerged the northern hemisphere from Europe to Asia. China also has such legends, and Western countries also have such legends.