Oshino Hakkai, literally means the Eight Seas of Oshino, is a set of eight ponds in Oshino, a small village, located between Lake Kawaguchi and Lake Yamanaka. The eight ponds are fed by snowmelt from the slopes of Mount Fuji. The snowmelt is filtered down the mountain through porous layers of lava over a period of several decades, resulting in very clear spring water.
The Edo period lasted for 265 years from 1603 to 1868. The first Shogun of the Tokugawa Shogunate was Tokugawa Ieyasu. Ieyasu unified the fragmented pieces of Japan to one country. Before the Edo period, there was a long period of civil war in feudal Japan.
Before his death Ieyasu had instructed his retainers to bury his remains at Mount Kuno. Ieyasu’s successor, the second Shogun, Tokugawa Hidetada carried out Ieyasu’s instructions and ordered a shrine to be built. A master carpenter named Nakai Masakiyo was chosen to build the shrine. He built an elaborately decorated shrine using the style of Gongen-zukuri (https://en.wikipedia.org/wiki/Ishi-no-ma-zukuri). The shrine became a model for other Tosho shrines all over Japan, including the Nikko Tosho Shrine.