Info below courtesy of guest reviewer Cami.
This sauna is located in the basement of the hotel, and easy to find, it is probably the most famous of the Asan/Onyang hot springs, certainly the oldest. Admission is is 6000 w. It is smaller than Asan Spavis or Asan Hot Springs, more of a traditional hot springs and it got pretty crowded. The locker room has a few large lockers, you can ask for one of them if you have a lot of stuff. There are also massage chairs in the locker room. In the back of the locker room is a massage area where they offer different treatments, this was closed when I went because it was evening.
Inside the sauna is a warm tub, a hot tub (which was actually too hot), an event pool of mugwort and a large cold pool. There is a skin scrubbing room also and a wet and dry sauna. Outside is a rocky outdoor pool, which is warm, and the best thing is the waterfalls on the side. I didn't notice them until another lady turned them on, but they feel fabulous! Also outside is a small bulgama, but it wasn't very hot.
This place is supposed to have the best water, and I must say, it does! The facilities are not as nice as some newer spas, but that water is great!
Onyang Hot Spring (Tourist) Hotel
충남 아산시 온천1동 242
Chungnam-do, Asan-si, Oncheon 1-dong 242
Wikimapia of Onyang Hot Spring (Tourist) Hotel