Kawayu Onsen (Wakayama)

The Kawayu Onsen hot spring is located in Tanabe City in southern Wakayama Prefecture. The hot springs around the Otogawa River, a tributary of the nature-rich Kumanogawa river. Kawayu Onsen is a rare spring in Japan in that it naturally mixes the boiling, 70 degree (C) water rising from the river bed, with the cooler running water of the river.  While the hot spring town along the river has plenty of bathhouses and hotels, Kawayu Onsen allows you to dig your own open-air bathtub -- an experience that you can't find elsewhere.
From December to February, the river is dammed up to create the the "Sennin Buro (Thousand-person bath)".  With a width of 40 meters and length of 15 meters, it is said to be the largest open-air onsen in Japan. The "Sennin Buro Karuta Tournament" is held annually in January, in which bathers fight over cards in a game of Karuta (A traditional Japanese card game) in the enormous bathtub.
In addition, an illumination event called "Yukemuri Toro (Steam Lanterns)" calls in January lights up the Sennin Onsen. Dipping in the hot spring water, in the crisp night air, while looking up at a sky-full of stars is a whole new luxurious experience.