In 1986, local dive operator Kihachiro Aratake was not searching for lost civilizations. He was looking for hammerhead sharks.

The waters around Yonaguni are famous among divers for strong currents and pelagic life. While scouting new spots off the southern coast of the island, Aratake descended along a familiar slope and suddenly saw something that did not fit with the random texture of the seafloor. The rock below him dropped away in wide steps, with straight edges and sharp corners that looked more like abandoned architecture than natural reef.

What he had found was a massive sandstone formation roughly one hundred and fifty meters long, forty meters wide, and about twenty seven meters tall. Its most striking feature was a stack of broad, flat terraces rising like a stepped platform from the seabed. Each ledge seemed to connect to the next through what looked like stairs and narrow landings. Seen through a diver's mask in clear water, it resembled a ziggurat that had slipped quietly beneath the waves.

Word spread quickly. Photographs of divers dwarfed by giant steps made their way into magazines and documentaries. The site acquired several names depending on who you asked: the Yonaguni Monument, the Yonaguni Submarine Ruins, or simply the Yonaguni Submarine Topography. Whatever the label, the core question was the same. Was this an extraordinary example of natural erosion, or the remains of something deliberately carved?

The Case for Human Hands

One of the strongest voices arguing for human involvement has been marine geologist Professor Masaaki Kimura of the University of the Ryukyus. Kimura has spent years returning to the site, logging more than a hundred dives while mapping and measuring the formation.

From his perspective, there are simply too many straight lines and right angles to be comfortable. He points to flat planes that meet at near perfect corners, long linear features that resemble roads or channels, and surfaces that look as if they have been cut. In interviews and papers, he has described what he interprets as tool marks and quarry traces, places where blocks may have been removed. He has even mentioned shapes that, to his eye, resemble animals or human faces carved into the rock.

Kimura does not see just one monument. He has spoken of an entire complex. In his reconstructions there are pyramidal platforms, castle like masses, broad plazas, possible roads, and retaining walls that seem to frame and protect the central structure. Beyond Yonaguni itself, he has identified additional formations off Okinawa that he believes are related, extending the footprint of this hypothetical culture across a wide area of the seafloor.

At first he proposed a very ancient origin, on the order of ten thousand years, when lower sea levels would have left the terraces above water. Later, he suggested a more conservative window of two to three thousand years, with tectonic shifts lowering the structure after it was built. In both versions, though, there is an underlying assumption. Someone, at some point, intentionally shaped this landscape.

Skeptics push back hard on that assumption. Many geologists see in Yonaguni not a lost city, but textbook examples of how layered sandstone behaves when subjected to stress, fracture, and erosion. Natural jointing in the rock can produce long, straight cracks. Repeated wave action and currents can break those layers into blocks. Given enough time, you get steps, ledges, and pillars that look almost artificial to a human brain that is wired to spot patterns.

They also point out what is missing. There are no clearly documented artifacts recovered in situ, no ceramics, tools, or unambiguous carvings that can be reliably tied to the formation rather than to nearby sites. Official archaeological bodies in Japan have not recognized Yonaguni as a man made ruin, largely because the conventional evidence they would need is not there.

Between these two positions, the dive site continues to draw people who want to see it with their own eyes. Some descend with measuring tapes and cameras, trying to catalogue every edge. Others simply hover over the terraces in the surge, aware that they are moving through a place that might be nothing more than an exceptional arrangement of rock, or might be the collapsed surface of a story that has not yet been written down.