Kusatsu Onsen Yubatake

Gunma's hot springs range from quiet neighborhood bathhouses to destinations in their own right — Kusatsu Onsen Yubatake is closer to the latter.

Kusatsu Onsen (草津温泉) is Japan's most famous hot spring resort, renowned for its yubatake (湯畑, 'hot water field') — a steaming wooden channel system where 32,000 liters of 55-65°C mineral water flows per minute, cooling from volcanic source to bathable temperature. The yubatake sits in the town center, billowing sulfurous steam day and night, surrounded by traditional inns (ryokan), souvenir shops, and public bathhouses. The water is highly acidic (pH 2.0-2.1), giving it powerful antibacterial properties and a distinctive sulfur aroma.

Kusatsu's hot spring culture dates to the 12th century, and the town has preserved its onsen machi (温泉街, hot spring town) character — wooden buildings, stone paths, and the ritual of yumomi (湯もみ, water-stirring performance) where performers use long wooden paddles to cool bath water while singing traditional songs. The town is perched at 1,200m elevation in the Joshu Mountains, making it a year-round destination — summer hiking, autumn foliage, winter skiing, and spring renewal.

The wooden channels of the yubatake exist because the spring water arrives too hot and too acidic to bathe in directly. Historically, cooling the water fast enough to make it usable meant either diluting it — which weakens its mineral concentration — or physically stirring it to release heat, the origin of the yumomi paddling technique performed at Netsunoyu today. The lattice of wooden troughs visible at the yubatake serves the same cooling function on a larger scale, letting the water lose heat as it flows through open air before reaching the town's bathhouses and ryokan.

Kusatsu's spring water originates from volcanic activity in the surrounding mountains, and the extreme acidity that makes it so effective against bacteria also historically limited what could grow or survive in the yubatake channel itself — a fact locals point to when explaining why Kusatsu earned its reputation as a purifying, almost medicinal hot spring rather than simply a relaxing one. This reputation supported a long tradition of toji, extended therapeutic bathing stays that could run a week or more, in which visitors soaked multiple times a day over consecutive days rather than bathing once during a short visit — a practice that shaped Kusatsu's ryokan culture around longer stays even as most modern travelers now visit for a night or two.