Senninburo

Each winter, between December and February, the stony bed of the Otaigawa River at Kawayu Onsen (Wakayama) is excavated to create a vast open-air communal bath called the Senninburo ("Immortals' Bath"). Hot spring water at 73 degrees Celsius seeps up naturally from below and cools to bathing temperature where it mixes with the river. The scale changes every year depending on conditions, and the edges you dig yourself define your spot. Swimsuits and towels are required here: this is a riverbed, public and wide open, and the rules exist accordingly. The name suits it: lying back in the water under a winter sky, the place feels genuinely otherworldly.