Kusatsu Onsen, in the mountains of Gunma Prefecture, is one of Japan's highest-volume hot spring sources, discharging roughly 4,000 liters of spring water per minute. At the town's center, the Yubatake ("hot water field") channels steaming sulfuric water down wooden troughs in an iconic, ever-present cloud of steam. The water is strongly acidic, around pH 2, prized since the Edo period for its antiseptic properties, giving rise to the old saying that Kusatsu cures every illness except lovesickness. The traditional yumomi performance, in which attendants stir the water with long wooden paddles to cool it before bathing, is still staged for visitors today.
