Gunma Prefecture is filled with onsen, but none quite like Manza Onsen. Manza is incorporated into Tsumagoi Village, but is situated high on the slopes of Mt. Kusatsu-Shirane. Getting there is a challenge, requiring a journey up twisting roads and along the volcanic caldera of the mountain. Manza Onsen is situated approximately 1,800 meters above sea level, making it the highest elevation onsen town in Japan.
Those who make the journey are treated to stunning views, of both the wide mountain ranges in the distance, and the Mars-esque landscape of Kusatsu-Shirane. In the summer, Manza's unique locale offers sweet relief from the heat down below. In the winter, Manza is fiercely cold and piled with snow, perfect for skiing and snowboarding.
The first thing you notice in Manza is the smell. The area is full of fumaroles venting sulfuric steam from the geothermal springs. The water for the baths in Manza Onsen is extremely acidic, and has been called the most concentrated hot spring in Japan. As a result, the smell of sulfur is everywhere. Don't worry though, you'll get used to the smell pretty quickly. There are some excellent paths along the yubatake to view the vents, as well as get a good look at the water being pumped up from the streams.
Manza Onsen's water is a unique, milky color that's opaque in pools.The trails in the area go along the stream feeding into the town's bath water supply, and have several fantastic viewpoints. You can also explore Kumashiro Cave, a cave that's home to an ancient shrine, and where several pieces of prehistoric pottery were unearthed. However, these trails are inaccessible in the winter months.
All of the hotels in the area offer day-bathing options. You can choose your bathing experience - from the fancy resorts to the humble local ryokans. Several hotels also offer konyoku (mixed gender) baths. You can check the bath options, and day bathing prices for all the hotels in Manza Onsen on the town's webpage. If you want to explore several baths, you can purchase a wooden yumeguri (onsen tour) amulet that will give you discounted access for all the onsen in town. All of the baths are filled with Manza's distinctive mineral-rich water. The milky water will literally make your body disappear when you enter, and is supposed to be excellent for your skin and digestive health.
Before making your way to Manza Onsen, be sure to check the volcanic activity on Mt. Kusatsu-Shirane. Roads can sometimes be closed, or have limited access, depending on these conditions.