The Macaque have their own private onsen (hotspring).
Trivia: the three monkeys who "hear no evil, see no evil, and speak no evil" were based on the Japanese Macaque. The three truths of the Tendai sect of Buddhism are Mizaru (see no evil), Kikazaru (hear no evil), and iwazaru (speak no evil). The Japanese verb ending-zaru and the word for monkey (salu) are similar in sound. Perhaps the use of the monkeys represents religious symbolism.
I am not sure I would drink the water...
Excuse us Mrs. Haegen!
For more information see:
http://www.jigokudani-yaenkoen.co.jp/english/top/english.html