Yonaguni monument: man-made city or nature's game

Yonaguni lies about 100 km east of Taiwan. The island is considered the westernmost inhabited island of Japan (according to the monument's Wikipedia page). Off its southern coast, on the seafloor, lie stone terraces with smooth ledges. The debate over who created them has been going on since 1986 and is still unresolved.

In the spring of that year, diver Kihachiro Aratake was looking for hammerhead sharks in these waters and noticed a massive structure with regular shapes (story of the discovery). Aratake was the director of the Yonaguni tourism association, so he quickly brought scientists to the site. The underwater ruins were dubbed the Japanese Atlantis.

Science has no definitive answer. Neither Japan's Agency for Cultural Affairs nor the Okinawa Prefecture authorities have recognized the site as a monument (official status).

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Key facts about the Yonaguni Monument

💡 In short:

• The monument lies off the island's southern coast at depths of up to 26 m (monument depth).
• The base is made of sandstone and siltstone of the Yaeyama Group, about 20 million years old (rock description).
• We verified the depth, geology, and discovery history in 09/2026 using the monument overview.

The stone structures cover a large area almost around the entire southern coast. The central part is what is called the monument: it rises from the seafloor in steps to a flat platform. Similar ledges exist on the shore itself, which is why geologists have argued for years about the nature of these formations. When viewed from above, the shape reads as a staircase.

How the underwater terraces were discovered

Aratake was diving off Cape Arakawabana at the southern tip of the island. In winter, schools of hammerhead sharks come here, and divers from all over Japan follow them (dive guide). In 1986, Aratake was looking for a good spot to observe the sharks and instead found a stone mass with sharp edges (the same discovery).

Soon a team of scientists led by Masaaki Kimura from the University of the Ryukyus arrived at the site (research team). The discovery quickly spread beyond academic circles: newspapers wrote about it, and later films were made.

What the monument is

The main structure sits beneath the Arakawabana cliff, with its front face oriented south-southeast (site description). The top of the platform lies at a depth of about 5 m, while its base extends down to 26 m. The walls and steps are made of early Miocene sandstone and siltstone.

The rock belongs to the Yaeyama Group and was deposited roughly 20 million years ago (monument geology). Most of the blocks are connected to the main rock mass rather than assembled from separate stones. Long channels adjoin the platform, which proponents of the man-made theory call roads.

Several other objects were found around the central mass. One resembles a wall of large boulders, another looks like a narrow passage between rocks. It is these shapes that sparked the debate about a sunken city.

Natural formation: the geologists' view

Geologist Robert Schoch of Boston University considers the monument natural (Schoch's analysis). The sandstone here is broken by parallel bedding planes and vertical joints, along which the rock easily splits into rectangular blocks. The region is seismic, and earthquakes regularly fracture the stone.

According to Schoch, the "walls" turn out to be simply platforms that ended up vertical after erosion of the rock beneath them, and the "roads" turn out to be natural channels. He calls the "carvings" on the stones natural scratches. Researcher John Anthony West, who examined the site together with the geologist, agreed with him.

German geologist Wolf Wichmann studied the monument in 1999 during a Spiegel TV expedition and returned in 2001 at Graham Hancock's invitation (study history). He also concluded that it was natural in origin. Similar stepped rock formations exist on the Yonaguni shore at a site called Sanninudai.

In 2019, Japanese researchers analyzed the island's topography and rock layers (Ogata's paper). Their conclusion is simple: the monument looks like a structure, but it was created by weathering and erosion along the sandstone's bedding planes and joints.

Man-made city: Kimura's view

Masaaki Kimura dived at the monument for many years and insists on its man-made origin (Kimura's arguments). He identifies a pyramid, castles, roads, and a stadium in the shapes. Initially the geologist estimated the structure's age at a minimum of 10,000 years, when sea levels were lower.

In 2007, at the Pacific Science Congress, Kimura revised his estimate to 2,000-3,000 years (2007 paper). In his view, the city was built and later submerged due to tectonic movement. He links its builders to the Yamatai culture.

Kimura describes a stone-carved sculpture with long arms, traces of carving, and holes in the rocks. Proponents of the man-made theory, including Graham Hancock, point to the lack of loose debris on the flat areas. Working against this theory is the strong current, which carries away loose material.

Diving at the monument

The dive site is usually done at a depth of about 25 m, and the dive follows the current (dive guide). Visibility often exceeds 30 m, and in winter schools of hammerhead sharks come close to the ledges (site description). The route is not difficult in terms of depth, but it requires experience because of the currents.

In summer the water is calmer, and the dive suits intermediate divers (seasons). In winter conditions are harsher, but the visibility and the shark encounters are worth it. Experienced divers go to the monument with a guide who knows the local currents.

If you plan to dive the monument, it is best to book dive tours on GetYourGuide in advance: in winter, during shark season, spots with local operators sell out quickly.

How to get to Yonaguni

Direct flights to the island are operated by Ryukyu Air Commuter from Naha Airport (flight to Yonaguni). There are one or two flights a day, depending on the season. The second option is a Japan Transocean Air flight to Ishigaki and then a local flight onward.

Traveling by sea is cheaper but longer. The ferry from Ishigaki takes about four hours (ferry to Yonaguni). This option works if you have time to spare and want to see the Ryukyu Islands from the water.

Tickets for local flights sell out in advance, especially during shark season. It is convenient to compare flights on Kiwi.com before booking accommodation on the island.

Yonaguni Monument on the map

The monument lies off the southern coast of the island, near Cape Arakawabana. You can open the map point right here.

⁉️🤔 Frequently asked questions

Was the Yonaguni Monument created by humans or by nature?

There is no definitive answer. Geologists Robert Schoch and Wolf Wichmann consider the formations natural and explain them as sandstone fractures. Masaaki Kimura insists on a man-made origin and sees a city in the formations. The official authorities of Japan have not recognized the object as a monument, and the debate has been going on since 1986.

At what depth is the monument?

The top of the platform lies at a depth of about 5 m, and the base goes down to 26 m. The dive site is usually done at a depth of about 25 m.

Who is the monument dive suitable for?

The dive requires experience because of strong currents. Experienced divers dive here year-round. Intermediate divers are better off choosing summer and going with a guide. In winter the current is stronger, but the chance of seeing hammerhead sharks is higher. Take a morning departure: there is less wind.

When is the best time to go to Yonaguni?

In winter, from November to March, hammerhead sharks come close to the island, and visibility stays high. In summer the sea is calmer, and diving is more comfortable for intermediate level. Both seasons work; the choice depends on your skills and your willingness to put up with the current.

How do you get to the island?

You can get there by direct flight from Naha or by ferry from Ishigaki. Tickets for both types of transport are best booked in advance, especially in winter.

In short: what to book

The Yonaguni Monument is interesting to both historians and divers. If you are going for the dives, plan for winter and book a guided dive tour. If geology and history matter more to you, one day on the island and a relaxed summer dive will be enough.

If you are short on time, choose a direct flight from Naha. If you want to save money, take the ferry from Ishigaki. If you are traveling in winter, book a dive tour on GetYourGuide in advance.

Related article: the ghost island of Gunkanjima.