Keya Otehon Sea Cave — Accessible Ocean Geology

On the western tip of Itoshima Peninsula, the Genkai Sea has been cutting into basalt cliffs for millennia, producing a 6-km stretch of dramatic geological formations including Japan's largest natural sea cave — Keya Otehon (大門), a 64-meter arch through which the sea flows in turquoise surges at high tide. The cave is accessible on foot at low tide along a sea-level path that requires scrambling over barnacle-crusted boulders.

The approach begins at Keya Beach, one of the few remaining unspoiled beaches in the Fukuoka metropolitan area, and continues along a coastal path past standing rocks, tide pools thick with sea anemones, and a small fishing harbor where boats launch before dawn. The cave itself is dark inside and resonant — waves entering from opposite directions create booming echoes.