Kawayu Onsen

A spa town on the rim of Japan's largest caldera

Kawayu sits in Teshikaga, on the northern rim of the Kussharo caldera — the largest in Japan — within Akan-Mashū National Park. The water is not piped from a distant source; it is fed from Mt. Iō (硫黄山, "Sulfur Mountain"), an active vent two and a half kilometres south of town, whose fumaroles still hiss yellow against black volcanic rock. Modern Teshikaga began with that mountain. Sulfur mining opened on Iō-zan in 1877 and ran on and off through the Meiji period, building the roads and rail that later carried the first bathers. The mine closed in the mid-1960s. The spa itself grew slowly: a single inn through the late Taishō years, then a proper onsen quarter from the Shōwa era onward, given its frame by the 1934 designation of Akan National Park.

Kawayu is also the hometown of Yokozuna Taihō (Kōki Naya), the 48th Yokozuna, who spent his school years in Teshikaga and went on to set a record for tournament victories that stood for decades. The small Sumō Memorial Hall in town keeps his trophies and ceremonial rope on display.

Sulfur in the gutters

The town's signature is its water, strongly acidic at around pH 1.6–1.8, sharply sulfurous, and abundant enough that the overflow from the inns runs in open gutters along the main streets. Steam vents in the road shoulder release it into winter air; free foot baths sit at the corners. The smell of hydrogen sulfide is constant, the sound of running water nearly so. Several inns offer the source water with no recirculation.

Winter is the season. Kussharo freezes along its shore, frost-rimed trees crowd the lake, and whooper swans overwinter in the open patches kept warm by ground geothermal flow.

Beyond town

A short drive down to the lakeshore reaches Sunayu, the "sand hot spring" beach where hot water seeps up through the lake sand and visitors dig their own footbath with a borrowed shovel. The graded trails at the foot of Mt. Iō lead within metres of active vents encrusted in yellow sulfur crystals. Farther east, the caldera wall opens onto Lake Mashū, famed for its clarity and its standing summer fog.