Yonaguni Monument

The formation is bedded sandstone and mudstone in a tectonically active zone. That lithology fractures along parallel bedding planes and near-perpendicular joint sets, producing flat surfaces and right angles without assistance. Comparable stepped formations occur above water on Yonaguni and neighbouring islands, where nobody proposes a builder.

The structure is also continuous with the surrounding bedrock. Cut-stone architecture generally is not: it consists of separated, moved units. Here there are no detached blocks, no quarry debris, no mortar, no fill.

A minority position, associated primarily with a Japanese marine geologist who has dived the site extensively, identifies features it argues are anthropogenic: paired holes, a supposed carved face, channels that appear to drain rather than accumulate. These have not been demonstrated to exceed what jointing and bioerosion produce, and no artefacts, tools, or occupation material have been recovered from the site.

For the formation to have been worked while dry it would need to date to the last glacial low stand, roughly ten thousand years ago or more. That is not impossible for the region — Jōmon-era populations were present in the archipelago — but it would require a monumental stone tradition for which the terrestrial record on those islands provides no support.