Oshino (忍野) is a small village in Yamanashi Prefecture. The village used to be a lake called Lake Utsu (宇津湖). Lake Utsu used to be the biggest lake surrounding Mount Fuji. When Mt. Fuji erupted around 800 AD, the lava from Mt. Fuji split the lake into two, Lake Yamanaka (山中湖) and Lake Oshino. Later, the lake Oshino dried up, but the eight spring (pond) still remain and they are called Oshino Hakkai (忍野八海).
The spring waters are underground water that flowed down from Mt. Fuji. When the snow melt into water from the top of Mt. Fuji, it got filtered down the mountain through layers of lava, traveling for over 80 years to finally spring at Oshino Hakkai, resulting a very pure, clear water.
The purity of the water is the proud of the locals. It is said that the water is one of the best waters in Japan.