In the shimmering blue waters off Japan's Yonaguni Island lies a geological wonder.
With its top peaking just 6 meters (20 feet) below sea level, extending down to a depth of 24 meters, the Yonaguni Monument looks, for all the world, like a vast, ruined citadel, a remnant of an ancient civilization drowned by the waves.
According to most geologists, of course, it's no such thing; its stepped sandstone and mudstone form is understood as a natural creation, shaped along fractures and bedding planes by tectonic stresses and relentless erosion.
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The formation was discovered in 1987 by diving instructor Kihachiro Aratake, and quickly attracted the attention of geologists. It was, in a nutshell, unlike many other geological formations familiar to scientists, especially in its scale and unusually ordered appearance.
It consists of large slabs of stone, arranged in a way that resembles steps or terraces, with crisply squared edges and corners – shapes that are uncommon in natural settings at this scale, which have invited comparisons to stepped pyramids or ziggurats.
So striking was the appearance up close that geologist Masaaki Kimura of the University of the Ryukyus spent several years compiling a detailed argument that the structure had been modified or constructed by human hands, before being submerged by rising seas around 10,000 years ago.
This view is highly controversial among his fellow geologists.
While relatively few peer-reviewed studies have focused directly on the Yonaguni formation, a broader body of geological evidence suggests that its strange, structured appearance can be explained by natural processes acting over thousands of years.
We know that our planet can make some remarkably geometric rocks.
The hexagonal columns of Ireland's Giant's Causeway and Scotland's Fingal's Cave are, literally, the stuff of legend.
The Tessellated Pavement of Tasmania, Australia, looks like neatly laid paving stones at the edge of the ocean, while the Al Naslaa rock of Saudi Arabia is split by an astonishingly clean, straight fracture.
Meanwhile, Preikestolen in Norway – Pulpit Rock – is famed for its sheer, flat geometry.
There are several natural geological features and processes relevant to the Yonaguni formation.
A bedding plane is a natural layer in sedimentary rocks like sandstone and mudstone, the boundary where two deposition epochs meet, separating layers of rock with different properties. These are often flat planes and a natural point of weakness in a rock formation.
Perpendicular to these, rock formations can develop joint sets. These are fractures in the formation, often parallel to one another, that split open when the rock is stressed – under tremors, for example – separating the rock into surprisingly neat blocks.
As noted by geologist Robert Schoch of Boston University, who dived at the site in 1997, "Yonaguni lies in an earthquake-prone region; such earthquakes tend to fracture the rocks in a regular manner."
Because Yonaguni lies in a fault zone, it experiences significant tremor activity that could easily explain both the regularity of the fractures and the stepped formation.
When the ground shakes under the formation, the rocks break and slip away from each other at these natural weakness points – activity that can produce the shape of the Yonaguni Monument.
Meanwhile, the constantly moving ocean currents erode the fractures, separating the rocks from each other and scouring the surfaces flat.
Schoch also noted that nearby rock formations on Yonaguni Island, although rounded and more strongly eroded, were arranged in a similar manner to the underwater formation.
"Though the slope itself, now a tumult of ragged, fractured planes," wrote the late author John Anthony West, who explored with Schoch, "did not much look like the underwater formation we'd been studying, it was clear enough that it was basically the same geomorphology – just that the slope, exposed only to wind and rain, had taken on a very different and ragged appearance over thousands of years."
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Because underwater geology is difficult and expensive, and everything about the Yonaguni Monument and its surrounding geology can be explained by natural processes, more detailed surveys of the site are yet to be undertaken.
However, "While these formations were once thought to be artificial, no archaeological remains or traces of human activity have been found," noted a team of geologists led by Hironobu Suga of Kyushu University at the 2024 Association of Japanese Geographers Spring Academic Conference.
"Through underwater observations, we were able to observe erosion processes, such as bedrock detachment, abrasion, and gravel generation, as well as the ongoing formation of erosional formations, such as potholes of various shapes and sizes.
"These findings suggest that the ruin-like formations are being created through the continuous weathering and erosion of sandstone on the seafloor."
And honestly? The fact that Earth can create such dazzling structures just through time and jiggling is fascinating enough.