Jigokudani Monkey Park is in Nagano Prefecture, Honshu, Japan. It is part of the Joshinetsu Kogen National Park, and is located in the valley of the Yokoyu-River, in the northern part of the prefecture. The name Jigokudani, meaning "Hell's Valley", is due to the steam and boiling water that bubbles out of small crevices in the frozen ground, surrounded by steep cliffs and formidably cold and hostile forests.
The heavy snowfalls (snow covers the ground for 4 months a year), an elevation of 850 metres, and being only accessible via a narrow two kilometre footpath through the forest, keep it fairly uncrowded despite being relatively well-known.
It is famous for its large population of wild Japanese Macaques (Macaca fuscata), more commonly referred to as Snow Monkeys, that go to the valley during the winter, foraging elsewhere in the national park during the warmer months. It was discovered in 1963 that the monkeys would descend from the steep cliffs and forest to sit in the warm waters of the hot springs, and return to the security of the forests in the evenings. The monkeys are encouraged to continue this behaviour by feeding being provided in and around the hot spring pool.
See http://www.jigokudani-yaenkoen.co.jp/english/html/top_e.htm for more information.