From Akita Wiki
Okuokuhachikurō Onsen (奥々八九郎温泉 or 奥奥八九郎温泉) is an onsen located in the mountains in Kosaka. In 1974 while exploratory drilling for kuroko (黒鉱; kurokō) deposits at a depth or around 320-350m onsen water was discovered. When drilling was completed a valve was closed to stop the water. Around 1984 that valve was reopened and the onsen water began to flow. The onsen is out in the open beside a dirt trail in the mountains. There are 4 baths all of which are mixed bathing and there are no facilities. It is completely free to use.
Take route 282 north out of the town of Kosaka, towards Hirosaki. Turn right at the Asunarō （あすなろう） sign. Follow the curving road through the collection of houses until the point where the road splits into a sharp left and a dirt road that continues straight through a forest. This dirt road will eventually lead you to the onsen itself, after approximately 3 km.
Though the onsen itself remains (more or less) accessible in winter, the dirt road is impassable by car. Bring snowshoes or skis if you plan on a winter bathing experience.