Glaciers in Manhattan
Cities sinking beneath the waves are nothing new, and this has occurred throughout history. Even Cleopatra's palace in Alexandria is on the ocean floor. In prehistory, consider that glaciers once covered temperate latitudes only 12,000 years ago. All the glacial ice extant today doesn't even amount to 1/10th of that, and we see signs of underwater cities and settlements throughout the world from that period. Even today, 90% of the human population lives along the seacoast, and migration has always ebbed and flowed with the ever changing coastline.