We visited Korakukan onsen because it is is located inside the Jiokokudani nature reserve, which is populated by a large troop of wild Japanese macaque monkeys or 'snow monkeys'.
Video: monkeys in the snow
Jiokokudani is famous as the only place in the world where monkeys get into the hot-springs to warm-up (as you can read at the official website). It is very popular with american tourists. All day long a steady stream tramp along the forest path, many with shell-shocked "this is much further / colder than I expected" expressions.
This is the monkey's hot-spring bath, surrounded by tourists.
Video: Snow monkeys in the hot-spring bath. Notice the pairs of monkeys grooming each other.
They say that if you carefully observe their behavior for long enough, you will have a profound sense of how little difference there is between you and the monkeys.
Watching this monkey scratch his bum, Daddy had to agree.
While we were taking photos of the hot-spring, the monkeys suddenly got-out and raced to pick-up food that was being scattered in the snow by the staff of the nature reserve. So I realized that the monkeys are not exactly 'wild' but rather 'salaried' or perhaps 'tenured'.
Thinking about it further, I suspect the reason this is the only place in the world where monkeys get into the hot-spring is because the monkeys pee and poop in the water. The hot-spring would soon become a health-hazard unless it was regularly cleaned out - probably by the same staff who distribute the food.
So perhaps Jiokokudani is a beautiful example of natural symbiosis: tourists visit because the monkeys sit in the hot-spring. The monkeys can sit in the hot-spring because it is cleaned by the nature-reserve staff. And the nature-reserve staff clean the bath because the tourists visit.