Keya no Oto Sea Cave
The largest sea cave in Japan, carved 64 metres high and 10 metres wide into the volcanic basalt coast of the Itoshima Peninsula. At high tide, waves boom against the walls with a sound like thunder (oto means sound). A 1-km clifftop trail leads to the cave entrance where you can stand at the lip and look straight down into crashing surf.
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Japan's largest sea cave, accessible by boat — the interior amplifies the sound of waves into a resonant roar ('oto' means sound). The tour takes about 20 minutes and is weather-dependent; ring ahead to check conditions. The Genkai Coast above has pleasant clifftop walking. Combine with Itoshima Peninsula for a full western Fukuoka coastline day.
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