Complex sentence: As time passed, mankind slowly forgot there were ever continents or land-based societies, the new mythology being that the world was created in a "great flood"; many, however, clung to the belief in a mythical place called "Dryland".
Split into simpler sentences: As time passed, mankind slowly forgot about continents or land-based societies, the new mythology being that the world was created in a "great flood". Many, however, clung to belief in a mythical place called "Dryland".