Jigokudani valley is cold and receives heavy snowfalls each year. It's surrounded by steep cliffs and inhospitable forests. Monkeys can't usually survive in such a hostile environment. However, the Monkey's of this valley have adapted to the cold by taking hot baths in the naturally occurring hot springs of the valley.
Japanese macaque are sometimes called snow monkeys. They are the Northern most non-human primates.
Hot springs (onsen) are key to the survival of Japanese macaque living in cold climates. Troops of monkeys are territorial and will defend their hot spring from other monkeys. Japanese macaque are also known to try to intimidate humans (if they think they can get away with it).
Jigokudani Valley is commonly referred to as Jigokudani Monkey Park. It's well known but receives few visitors in winter — it's only accessible via a narrow snow covered 2 kilometer (1.2 mile) footpath.
Japanese macaque are also found in several remote regions of Tohoku (Northern Japan) such as the Shimokita Peninsula. People also enjoy onsen baths throughout this region.
Snow monkeys and humans generally don't share the same baths. Japanese macaque can be quite aggressive — humans tend to feel vulnerable when they're naked and the monkey's sense it. It's not fun to take a bath with a monkey.