The beautiful blue water of Shirogane Aoi Ike (白金青い池) is not a natural phenomenon, but is a by-product of creating a dam at the Biei River in Hokkaido. The dam was built to stop the volcanic mud from the erupting Mt. Tokachi reaching the town of Biei in 1988. Aluminium from the cliffs seeped into the water and settled in the pond, resulting in this vivid blue hue.
