Awanoyu, Shirahone Onsen

Shirahone means white bone, and the name makes sense the moment you see the water: milky white and almost opaque, colored by magnesium and calcium sediment, with a silky feel. Awanoyu, an inn founded in 1912 in this small mountain village west of Matsumoto, owns the bath most people come for, a large mixed outdoor pool of that white water. Men and women change in separate rooms and meet in the pool.

Towels are allowed in the mixed bath: Japan Guide notes that women may wrap themselves, and the inn says guests bathe in thick bath towels and that women can step from their changing room straight into the water. The opacity does the rest. The temperature is lukewarm by onsen standards, so you can stay in a long time.

If you would rather not share, there are gender-separated indoor baths and a women-only outdoor pool. Day visitors pay 1,000 yen between 10:30 and 14:00 (last entry 13:30), and day bathing is closed on Wednesdays and Thursdays; the inn warns that extra closures can be added, so call the same day. If the inn is full, the village's public riverside bath costs 700 yen in season.

The real drawback is distance: without a car it takes two buses from Shin-Shimashima and up to two hours from Matsumoto, with only around four connections a day.