The Blue Pond outside the hot spring town of Shirogane Onsen is named after its deep blue color which it owes to natural minerals dissolved in the water. Not promoted as a tourist spot until recent years, the pond is part of an erosion control system to prevent damage to Biei in case of an eruption by nearby Mount Tokachidake.
This is the water of the Biei River and the water that contains sulfur and other hot spring elements such as sulfur contained in the tributaries that merge near Shirogane Onsen and “Shirohige no Taki” waterfalls, as well as aluminum. When these two waters mix, invisible “colloids” are formed. When sunlight collides with colloidal particles in water, light is scattered in various directions. It is said that blue light with shorter wavelengths is more easily scattered and thus reaches our eyes, making it appear blue. Another theory is that the white soil at the bottom of the pond reflects the light, enhancing the blue color.
PS: Flew in to Tokyo Haneda around 6.30 AM today and took another flight to New Chitose (Sapporo) landing around 9.30. By the time the baggage arrived and therental car was collected it took another one hour. Did sight seeing till just past 5PM , the time darkness sets in during the autumn.
This is the water of the Biei River and the water that contains sulfur and other hot spring elements such as sulfur contained in the tributaries that merge near Shirogane Onsen and “Shirohige no Taki” waterfalls, as well as aluminum. When these two waters mix, invisible “colloids” are formed. When sunlight collides with colloidal particles in water, light is scattered in various directions. It is said that blue light with shorter wavelengths is more easily scattered and thus reaches our eyes, making it appear blue. Another theory is that the white soil at the bottom of the pond reflects the light, enhancing the blue color.
PS: Flew in to Tokyo Haneda around 6.30 AM today and took another flight to New Chitose (Sapporo) landing around 9.30. By the time the baggage arrived and therental car was collected it took another one hour. Did sight seeing till just past 5PM , the time darkness sets in during the autumn.
Safe travels. 
