Have you read anything about the temple in Yonaguni Jaban? It has been estimated to be over eight thousand years old. As it was built while the land was dry, which was over eight thousand yaers ago.
Actually, I just finished reading about Yonaguni in a book called, 'Underworld'. Excellent read. They had a skeptical geologist dive with them on the site, and give all the possible natural explanations. But he never was able to rule out the possibility that they were man-made.
So it remains an open question, as far as I am aware.
Personally, I think it is a man-carved structure. But more research is necessary to establish that for sure.