Kusatsu Onsen

From Naganohara-Kusatsuguchi Station at the end of the JR Agatsuma Line, a bus climbs for twenty-five minutes through spruce forest and then crests a rise and drops into a wide volcanic plateau. Kusatsu sits in a bowl of its own steam. The Yubatake, the hot spring field at the center of town, covers an area roughly the size of a city block and produces water from over thirty separate vents simultaneously, the combined output exceeding 32,300 liters per minute. That figure makes it the highest natural hot spring output in Japan, and standing at the railing above the Yubatake with the steam rolling across the wooden channeling boards in front of you, the number becomes physically comprehensible.

The water temperature at the Yubatake vents runs between 50 and 94 degrees, which is too hot to bathe in without cooling. The traditional method for lowering it is called yumomi, and it involves a group of people standing in a line and stirring the water with wooden boards about 180 centimeters long in a coordinated motion, working the heat out by surface contact rather than diluting the spring with cold water. Cold water is not used because it would reduce the mineral concentration, and the minerals are the entire point. A performance of the yumomi ritual runs daily at the Netsu-no-Yu bathhouse in the town center, complete with traditional songs and the option for visitors to try the boards themselves.

The chemistry at Kusatsu is extreme. The pH sits between 1.7 and 2.1, making it one of the most acidic naturally occurring bathing waters in Japan, roughly equivalent to vinegar. The sulfur, aluminum sulfate, and chloride compounds dissolved in that water are present in such concentration that an iron nail placed in the bath would rust through in nine days. On skin, the effect is sharply antiseptic. There is a distinct tingle, like a mild exfoliant working, and the smell is sulfur without apology, thick and volcanic. After twenty minutes in a Kusatsu bath you step out with skin that looks slightly flushed and feels scrubbed clean from the inside.

The free public bathhouses, the most famous being Otaki-no-yu, Gozanoyu, and Sainokawara outdoor rotenburo, are open to all visitors with no reservation required. Sainokawara is the largest, a wide outdoor pool fed by water from the same fields, surrounded by stone walls and open sky. At night in winter, with snow on the surrounding rocks and the sulfur smell rising through freezing air, the heat of the water at pH 2 becomes something close to necessary. Kusatsu has been voted Japan's top onsen repeatedly in public surveys for years, and standing in that acid bath at midnight while it snows, the ranking does not feel like a surprise.