Tamatsuki Onsen is a popular sento open all night on Saturdays. Nice use of tile, baths are somewhat basic. There is a radon steam sauna and a radon salt bath that was not so hot that I enjoyed. The second floor sauna is amazing. Walking up the small narrow staircase the heat from the sauna gets you sweating before you even get to the door. That door is also TOO HOT TO TOUCH. I had to use my towel. Inside the tight L shaped room there's enough room for 4 or 5 people at most. The thermometer said the temperature was 115C! This was the hottest sauna I've ever been in and I doubt I'll find one as hot. Overall it was an interesting sento and worth checking out.