In February 2024 we took a winter detour from Tokyo up into the mountains of Nagano, to visit the only monkeys on earth famous for having their own onsen. Jigokudani means "hell valley", which felt dramatic until we saw the steam rising out of the frozen gorge and understood the name completely.
It is a long day out from Tokyo by train and bus, and worth every minute: come in winter, wear proper boots, and give yourself time to just stand there watching them soak. For the rest of that trip, start with our Tokyo post.