The Yubatake alone pours 5,000 litres per minute of hot spring water to the surface, and it is ranked as the most productive hot spring in all of Japan. The surface temperature of the water is more than 70 degrees Celsius, so it must be cooled before it can be used.
As the water bubbles to the surface, it is channeled into long wooden tunnels made of Japanese pinewood. Here, the water is cooled and minerals are collected (also known as 湯の花 "flower crystals"). The water comes to the bottom of the field into a pool where it is eventually pumped to the nearby ryokans and onsens.
The Japanese love their hot springs, and for good reason. The minerals from hot springs generally, and the Yubatake specifically purportedly cure everything except heartache. The yu no hana collected from the Yubatake are sold as powders, boasting the same medicinal properties as the water itself.
Wikipedia has a better picture of the entire enterprise.
We enjoyed our stay in Kusatsu. In addition to the onsening, we hiked around the Sai No Kawa Koen, shopped, and ate many delicacies. Below are some of the local favorites, in order: maitake with udon in a miso soy broth, maitake tempura, and sansai with soba.
In addition to the Japanese flavors, we ate some of the best (albeit more expensive) Korean cuisine. Korean pancake, a shot of ginseng liquor, grilled garlic, shrimp, and for the GP, some beef tongue.
My personal favorite activity (aside from the onsen and the eating) was to go to the foot bath cafe to enjoy a foot soak and a relaxing drink. This is located right next to the Yubatake, so you can't miss it.
There's even an English sign.
Foot spa bar is a perfect get away from a cold, rainy evening.
I elected for the honey ice cream. To my surprise, it also came with a frozen egg yolk. Tasty, yet strange.
We hit up the foot spa bar again the next day; this time I opted for matcha ice cream with "running bean"; this is ice cream more my speed. I have no idea what a "running bean" is in Japanese, except that it is a bigger, bolder, less sweet version of adzuki bean. Although it has the look and feel of a giant pinto, like its adzuki cousin, the running bean pairs nicely with the matcha and vanilla.
The little shopping area around the town center is quite lovely as well, and has many interesting shops, including a place to buy eggs boiled by the hot spring waters.
Just follow the lanterns and you will find your way to the shopping areas. They do tend to close early, about dinner time.
In addition to the onsens at your hotel and around town, you can go to the Sai no kawara koen (西の河原公園) where there is a great open air bath.
We planned to partake when it got a little darker because the men's side of the bath is quite exposed to the hikers in the upper levels of the trail. As one Tripadvisor reviewer put it:
"The men's bath is exposed. EXPOSED. There is a 2-3m wall surrounding the bathing area but there is also a public walking path that goes uphill a few meters from the wall. Once people get sufficiently high up the path they will be able to see everything. EVERYTHING. EVERYONE. ... The moment you get up and walk toward sthe changing room, you're flashing. What an experience."
Accordingly, if you are not comfortable being nude and viewed, perhaps stick to the onsens in the town. Below is a view of the men's side. The reviewer was indeed correct. Maybe in the summer, the vegetation is more lush, but during the rest of the seasons, the entrance to and egress from the pool could be more than daunting. I wish I had a better shot, but there was a guy in the pool. I didn't want him to think I was a creepy pervert Westerner, so I took pictures behind the trees.