"KUSATSU" ONSEN is one of the top 3 ONSENs in Japan, in term of the quality of the original hot-spring source, other than "ARIMA ONSEN" in Hyogo prefecture and "GERO ONSEN" in Gift prefecture.
The main reason why KUSATSU ONSEN is so popular is that there are several strong flows from the original hot-spring sources, which are benefitted by the surrounding highland mountain areas. There are total 6 main original hot-spring sources there, which are split into many ONSEN RYOKANs in Kusatsu.
You can also obtain a "Free Pass" to move around and try out to take different baths at 15 ONSEN INNs associated together. You can enjoy different hot-springs with different feelings and atmosphere on a same day.
The specific characteristic on KUSATSU ONSEN is a strong ACIDITY, which has a good benefit for "Skin problem", Diabetes and Neuralgia, since a several hundred years ago.
In term of logistic, you can take JR bullet-train (Shinkansen) from Tokyo Station to Takasaki stataion and then change to AZUMA Line to Haneo station. It takes about 3 hours including 15-min taxi from Haneo station to Kusatsu Onsen town.