白山洞门，土佐清水

Hakusan Natural Arch is a coastal rock formation in Tosashimizu City, Kochi Prefecture, on southern Shikoku Island's Cape Ashizuri. The arch formed through millions of years of wave erosion on sedimentary rock layers. Continuous wave action exploited rock weaknesses, gradually creating a tunnel until the arch shape emerged. "Hakusan" (White Mountain) likely refers to the white rock or cliff appearance. The arch is a natural landmark representing coastal geology in the Ashizuri-Uwakai National Park region, where rugged cliffs, sea caves, and formations characterize the Pacific Ocean-shaped landscape.

Hakusan Natural Arch demonstrates erosional coastal landform development and ongoing geological processes on Japan's southern coastlines. The arch evidences ocean erosive power on sedimentary layers over extended timeframes, part of broader Cape Ashizuri geology including marine terraces, sea cliffs, and erosional features from tectonic uplift and wave action. Designation within Ashizuri-Uwakai National Park recognizes natural and scenic value. The coastal environment supports marine ecosystems and seabird populations.

Visitors view the arch from observation points along coastal paths or viewing platforms. Vantage points allow observation of full structure and surrounding landscape. Viewing depends on ocean conditions: calm seas provide clearer views of the opening, rough seas create dramatic wave action. Coastal walking paths allow exploring broader Cape Ashizuri including other formations, cliffs, and vegetation. The undeveloped site has limited facilities, requiring preparation for coastal conditions including wind and wave spray.

Hakusan Natural Arch is on Cape Ashizuri peninsula, approximately 2 to 2.5 hours by car from Kochi City. Limited public transport includes infrequent buses from Kochi requiring 3 hours or more. Rental cars provide practical access. From Nakamura Station on the Tosa Kuroshio Railway, the cape is 60 to 90 minutes by bus or car. Admission is free.