Claims › Megalithic construction › The underwater Yonaguni Monument is a human-made structure
The underwater Yonaguni Monument is a human-made structure
The claim: The stepped underwater rock formation off Yonaguni Island, Japan, was shaped by people when sea level was lower, before it was drowned after the last Ice Age.
Where the evidence stands: Marine geologist Masaaki Kimura has argued since the 1990s that it is a carved monument. Geologist Robert Schoch, after diving the site, concluded it is mainly natural, possibly modified. No dated human-made artifacts have been reported from the formation in peer-reviewed work.
Background
The formation has flat terraces, straight edges and right-angled steps in Miocene sandstone. Sea level was far lower during the last glacial period (about 134 m lower at its lowest), so the location would once have been dry land. The question is whether the shapes are natural jointing and erosion or human work.
What has been measured
Dates, measurements and documented findings, kept separate from interpretation.
- Global sea level was about 134 m lower at the Last Glacial Maximum and rose mainly between about 16,500 and 8,200 years ago. [3]
Proponents' main arguments and cited evidence
- Kimura cites straight walls, terraces, possible tool marks and features he identifies as roads and carvings. [1]
The mainstream position and its evidence
- Schoch reports that the sandstone's parallel bedding and vertical joints naturally break into steps, and similar shapes occur on the island's shore. [2]
What evidence would help settle it
Dated tool marks or artifacts in sealed contexts on the formation, or a detailed joint-pattern survey showing shapes natural fracturing cannot make.
Sites
Related claims
Sources
- Kimura, M. (2004). Ancient megalithic construction beneath the sea off Ryukyu Islands in Japan, submerged by post-glacial sea-level change. Oceans '04 MTS/IEEE. doi:10.1109/oceans.2004.1405617 proponent
- Schoch, R. M. Yonaguni (author's own account of his dives and geological assessment). https://robertschoch.com/yonaguni.html critique
- Lambeck, K. et al. (2014). Sea level and global ice volumes from the Last Glacial Maximum to the Holocene. PNAS 111. doi:10.1073/pnas.1411762111 data
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Sources checked against Crossref and publisher pages on 2026-10-22. This entry presents a debated idea neutrally: the proponents' case first, then the mainstream position, in our own words. "Open question" is not a judgment that the idea is true or false. Labels describe the claim, not the people who hold it. Methodology.