Diamonds are appreciated!
Rolling back the clock to a time when the overworld was inhabited, we find the ancient Monument Dwellers. The commoners were those who usually lived outside or far away from the Ocean Monuments in stone cities and villages on the coast of where one could see the monument stuck above the water. They got orange tattoos to try and scare the pirates who constantly raided their villages. Being the first to be wiped out by the pirates and floods, everyone who survived evacuated to the ocean monuments, never to be seen again. If it was the flood that ended them, the guardians turning on them, something else, or a combination; no one knows.