Nakadake Onsen is a true wild spring bubbling up at about 1,800 m on the mid-slope of Mt. Nakadake in the Asahidake range of the Daisetsuzan Mountains, Higashikawa Town, Kamikawa District, Hokkaidō. Perched along the Asahidake hiking trail, it’s known as a “sky onsen” enjoyed only by mountaineers.

The water is a milky, sulfurous spring, extremely hot at source. Because it’s too hot to enter directly, you must channel in snowmelt or stream water from nearby snowfields and ravines to cool it down to a safe bathing temperature. The bath itself is nothing more than a simple pool dug into the earth—just large enough for a handful of people—and there are absolutely no facilities (no changing rooms, no toilets).