芥屋の大門, Sea cave in Itoshima, Japan.
Keya no Ohto is a sea cave cut into the basalt cliffs along the coast of Itoshima, Japan. The rock walls and ceiling display natural columns with hexagonal and octagonal cross-sections, formed by ancient volcanic activity.
The basalt columns formed millions of years ago from volcanic eruptions that shaped this coastline. Over time, seawater eroded the softer rock away, leaving the distinctive columnar structures exposed.
The name Keya no Ohto translates roughly to "the great gate of Keya," which matches the shape of the cave entrance seen from the water. Visitors arriving by boat often say the passage feels like crossing a threshold into a different world of shadow and stone.
The cave can only be reached by boat departing from Keya port, with service running mainly during the warmer part of the year. Arriving early in the day is a good idea, as wind on the open water can pick up quickly and affect departures.
The cave holds the status of a natural monument in Japan, a designation rarely given to geological formations along the sea. This recognition points to how unusual the quality of the basalt columns is within the Japanese context.
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