Noboribetsu Onsen (登別温泉) is a serious onsen: you can smell the sulphur from miles away. While the town is small, there are countless springs here, sending up mineral-rich waters. The baths received great fame when the town was designated as a health resort for injured soldiers following the 1904–05 Russo-Japanese War and remain Hokkaidō's most famous onsen today.
The source of the waters is Jigoku-dani, a hissing, steaming volcanic pit above town. According to legend, this hellish landscape is home to the oni (demon) Yukujin. Don't worry: he's kind and bestows luck. You'll find statues of him around town.