Okuyagen [Hot Spring] - Okuyagen

Japanese: 奥薬研[温泉] - おくやげん

...Fishery processing, such as squid delicacies, is also popular. Yagen Onsen, located in the mountains at the northern foot of Mount Osore, is blessed with the beautiful valley of the Ohata River, and 2km further upstream is Okuyagen Onsen, formerly known as Yunomata Onsen, known for its open-air bath for 1,000 people carved into green tuff. Ohata is the terminus of the Japanese National Railways Ohata Line (now the Shimokita Kotsu Line), which opened in 1939.

From [Yagen [Hot Springs]]

…In 1965, a highway was completed, and hotels, resorts, and national campsites were built. 2km further upstream is Okuyagen Onsen (simple spring, 57°C). [Hiroshi Yokoyama]. …

*Some of the terminology that refers to "Okuyagen [Hot Spring]" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…イカ珍味などの水産加工も盛ん。恐山北麓の山中にある薬研（やげん）温泉は大畑川の渓谷美に恵まれ，さらに上流2kmにはかつて湯ノ股温泉と呼ばれた奥薬研温泉があり，緑色凝灰岩をくりぬいた露天の千人風呂で知られる。大畑は39年に開通した国鉄大畑線(現，下北交通線)の終点。…