Hirauchi Kaichu Onsen is an extraordinarily rare natural seaside hot spring located on the southern coast of Yakushima Island, a UNESCO World Heritage Site in Kagoshima Prefecture. Discovered around 1599, this remarkable onsen has been maintained by the local Hirauchi community for over 400 years. The bathing pools are submerged beneath the Pacific Ocean during high tide and only emerge for approximately two hours before and after each low tide, twice daily. The spring water is classified as an alkaline simple hot spring (pH 9.0) with a source temperature of approximately 46.5 degrees Celsius. The clear, colorless water has skin-smoothing qualities and is reputed to be effective for neuralgia, muscle pain, joint pain, and rheumatism. The open-air rock pools sit directly on the ocean shoreline, offering breathtaking panoramic views of the Pacific. In June 2024, separate changing rooms for men and women were constructed. Swimwear and underwear are strictly prohibited; bathers must use a bath towel wrap or traditional bathing garment. A voluntary contribution of 300 yen is requested.