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Science & Nature • Geology

Yonaguni Monument

The Yonaguni Monument is a submerged rock formation off Yonaguni Island in Okinawa Prefecture, Japan.

By Ethan Anderson| Reviewed by Olivia Bennett, Standards & Fact-Checking Lead| Updated July 26, 2026|2 min read| Fact-checked

The Yonaguni Monument is a submerged rock formation off Yonaguni Island, the westernmost inhabited island of Japan, in Okinawa Prefecture. Discovered by a local diver in 1986, it is noted for its sharp edges, terraces, and step-like appearance. While its origin is debated, most geologists interpret it as a natural sandstone formation shaped by erosion.

Location and discovery

The monument lies underwater near Yonaguni Island in the Ryukyu Islands of Japan. It was first reported by local diver Kihachiro Aratake in 1986 and later became a subject of scientific and public debate. The formation is located at depths ranging from approximately 5 to 30 metres below sea level.

Geology and appearance

The formation is composed of sedimentary rock, commonly described as sandstone and shale, with strong bedding planes and joints. These features help explain its straight edges, terraces, and block-like shapes. The host rock dates to the Miocene epoch, around 20 million years old.

Interpretations of origin

Some observers have proposed that the structure is a human-made monument or altered archaeological site. The prevailing geological view is that it is natural, shaped by erosion, tectonics, and underwater currents. The sharp angles and step-like surfaces are attributed to jointing and differential erosion in the sedimentary rock.

Because of its unusual shape, the monument is often discussed in relation to lost civilizations and underwater mysteries. It remains a notable diving site and a frequent subject in books, documentaries, and popular media.

Key facts

  • It lies near Yonaguni Island, the westernmost inhabited island of Japan.
  • Discovered by local diver Kihachiro Aratake in 1986.
  • Has flat surfaces, steps, and vertical faces that make it appear artificial.
  • Most geologists interpret the structure as a natural sandstone formation.
  • Also called the Yonaguni Submarine Ruins or Yonaguni-jima Kaitei Chikei.
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Canadian context

Canadian readers may know the Yonaguni Monument through geology, underwater archaeology, and travel writing, as it is often discussed as an example of how natural rock formations can resemble human construction.

Frequently asked questions

What is the Yonaguni Monument?
It is a submerged rock formation off Yonaguni Island in Japan, known for its geometric, stepped appearance.
Who discovered the Yonaguni Monument?
It is generally credited to local diver Kihachiro Aratake, who reported it in 1986.
Is the Yonaguni Monument natural or man-made?
Most geologists consider it natural, while some researchers and writers have argued for human involvement.
How deep is the Yonaguni Monument?
Sources describe it as lying roughly 5 to 30 metres below the sea surface.
What rock is it made of?
It is commonly described as sandstone and shale associated with Miocene-age rock units.
Why is it famous?
Its sharp angles and step-like surfaces make it look artificial, which has led to long-running debate about its origin.

References

  1. Wikipedia — https://en.wikipedia.org/wiki/Yonaguni_Monument
    Supports: Location, alternate names, discovery context, and general description of the formation.
  2. Encyclopaedia Britannica — https://www.britannica.com/topic/Yonaguni-Monument
    Supports: Discovery in the mid-1980s, dimensions, debate over natural versus human-made origin.
  3. Atlas Obscura — https://www.atlasobscura.com/places/yonaguni-monument
    Supports: Location near Yonaguni Island and ongoing debate about the formation.
  4. Scuba Diving — https://www.scubadiving.com/dive-japans-mysterious-yonaguni-monument
    Supports: Discovery by divers in 1986 and scientific consensus that it is natural.
  5. Esovitae — https://esovitae.com/the-yonaguni-monument/
    Supports: Geological interpretation as natural sandstone carved by wave action and jointing.