Not a sanatorium as such, but about 15 years ago I was flying from Moscow to Yuzhno Sakhalinsk, and the flight was diverted to Petropavlovsk-Kamchatskiy. We were taken by bus to what was quite a remote hotel out of town which had hot springs to bathe in the grounds. This was the middle of winter and I think the temperature was around -25C, and I did try out one of the springs which was really pleasant experience. This is me in the foyer, next to a large stuffed bear. We flew onto Yuzhno the following morning once the snow storm had abated.