Kusatsu onsen is considered to be one of Japan's greatest hot spring resorts. In fact, it has been ranked as the number one hot spring (onsen) in Japan for 13 years straight!
The hot spring is located in Gunma prefecture. The springs here naturally produce a large amount of water (a water yield of 32,300 liters or more per minute). In addition to being the number one ranked hot spring in Japan, Kusatsu is also the most bountiful. Why not soak your worries away in these heavenly hot springs?
It takes about 2 hours and 30 minutes by Shinkansen (bullet train), plus about 30 minutes by bus, to get to Kusatsu onsen from Tokyo station . It's a little far... but worth it. If you have a JR pass and want to enjoy soaking in a hot spring, I strongly encourage you to go there. Although a day trip to Kusatsu from Tokyo is possible, staying overnight will give you more time to relax and unwind.
The bus to Kusatsu arrives at a bus terminal just outside the center of the town. From the bus stop it is a short, leisurely stroll to the center of this hot spring town. There is an information desk on the first floor of the bus terminal so stop by to grab an English map and get some directions. The staff at the information desk were very friendly and helpful when we were there, and there were English versions of most of the maps and flyers.
The center of Kusatsu features a large area with wooden sluices and channels that are designed to help cool the water. The pungent smell comes from the high concentration of sulfur in the water. This means that the water at Kusatsu is very acidic, which gives it superb antibacterial and other disinfecting properties. This acidic water is famous for its health benefits, with the water reported to be good for muscle and arthritic pain, healing and even high blood pressure! The generous supply of water from the hot springs means that the inns and other hot spring facilities in Kusatsu are able to offer hot spring water direct from the source.
There are lots of hot spring inns and ryokan dotted around the central water fields. I recommend getting the special 3-in-one onsen ticket at Goza-no-yu, the hot spring inn located next to the stone stairs near the central water fields (see picture below). The 3-in-one ticket will allow you to visit three of the most famous onsen in Kusatsu for a discounted fee. The three onsens included in the ticket are Goza-no-yu, Sai-no-kawara outdoor bath and Otaki-no-yu. The postcard-sized ticket itself makes a nice memento of the trip as each onsen inn will stamp it as you enter.
Sai-no-kawara, the second onsen included in the 3-in-one ticket, features an extremely large outdoor bath. It is magical to relax in the hot water while surrounded by the trees and mountains. Unfortunately there are no showers at this onsen so I recommend coming here for a mid-day soak and going to one of the other onsen if you want to get clean before turning in.
For more information about Kusatsu onsen, see their official website.