Yonaguni Monument: The Underwater “Lost City” — Why Most Geologists Call It Natural

Off the northwest coast of Japan’s Yonaguni Island lies a striking underwater formation that, at first glance, looks like the ruins of a sunken city. With its highest terraces about 6 meters (20 feet) below sea level and its features descending to roughly 24 meters, the Yonaguni Monument presents broad, stepped slabs and crisp edges that invite dramatic comparisons to pyramids and terraces.

Discovery and Debate

The formation was brought to attention in 1987 by diving instructor Kihachiro Aratake. Its scale and highly ordered appearance prompted geologist Masaaki Kimura to suggest that parts of the site may have been modified or constructed by humans and later submerged by rising seas around 10,000 years ago. That hypothesis remains controversial.

Why Many Scientists Favor a Natural Origin

Most geologists interpret the monument as a product of natural processes. Field observations and comparative examples show how tectonics and erosion can produce remarkably geometric rock forms. Boston University geologist Robert Schoch, who dived at Yonaguni in 1997, emphasized that the region is earthquake-prone and that seismic fracturing can produce regular, blocky jointing in layered sedimentary rocks.

Key Geological Mechanisms

• Bedding Planes: Natural layers in sandstone and mudstone form flat surfaces that act as predictable planes of weakness.
• Joints and Fractures: Parallel fractures develop under stress (for example, during earthquakes), breaking rock into regular slabs and blocks.
• Tectonic Activity: Yonaguni lies in a fault zone where repeated tremors can fracture and offset rock along straight lines.
• Marine Erosion: Ocean currents and wave action exploit fractures, separate slabs, smooth surfaces, and form erosional features such as potholes.

Comparative Examples

Nature produces other geometrically striking formations: the hexagonal basalt columns of Ireland’s Giant’s Causeway and Scotland’s Fingal’s Cave, Tasmania’s Tessellated Pavement, the clean fracture of Saudi Arabia’s Al Naslaa rock, and Norway’s Preikestolen (Pulpit Rock). These analogues show that regular, architectural-looking rock can arise without human intervention.

Recent Surveys and Conclusions

Detailed underwater geology is technically challenging and expensive, so exhaustive surveys are limited. However, a 2024 team led by Hironobu Suga (Kyushu University) reported at the Association of Japanese Geographers conference that no archaeological remains or traces of human activity have been found. Their underwater observations documented active erosion processes — bedrock detachment, abrasion, gravel generation and variously shaped potholes — and concluded the ruin-like appearance results from continuous weathering and marine erosion of seafloor sandstone.

“These findings suggest that the ruin-like formations are being created through the continuous weathering and erosion of sandstone on the seafloor.” — Hironobu Suga et al., 2024

Whether or not human hands ever shaped part of the structure, the Yonaguni Monument remains a powerful reminder that geological processes — time, tectonic movement and the ocean — can craft landscapes that look deliberately architectural. The controversy also highlights the value of careful, multidisciplinary fieldwork to distinguish cultural remains from natural marvels.