Cheonan-Asan is a large twin city area found just south of Seoul in Chungcheonnam-do. There are many ways to get here, depending on where you're coming from. It can be reached by KTX, and the subway line makes travel easier between the cities. There are also local rail lines which stop directly in Cheonan and Onyang Oncheon station to make your travel more direct. (Thanks to the commenter below, Helen, for supplying more info.^^)
The Onyang Oncheon is known as one of the oldest and hottest (57 degrees C!) natural springs in Korea. The sauna in Onyang Tourist Hotel (온양관광호텔- they call themselves "Onyang Hot Spring Hotel") is well-known in the area of Onyang in Asan.
This is the place to go to soak, scrub, and slow down for only 5,500 won. It has a lovely feel to it, with a lot of sun shining in from the windows and outdoor bathing area. Some showers are a bit fussy with temperature, but there are plenty to choose from if you happen to be at a cold one. In the main bathing area, you'll find 4 baths: a large cold tub, a pretty hot crescent-shaped tub, the event tub, and a hot-spring fed bath. The event tub was filled with an Ecto-cooler green water (which I later found out was due to the herb, artemisia a.k.a. mugwort.) and smelled amazing!
The real gem, where I spent most of my time, is the outdoor bath. The area is enclosed by rock and feels more like a lagoon, complete with fake flowers flowing down the walls. In addition, there is a hot, dry sauna room (bulgama) located outdoors as well but beware: they are serious about DRY. Bring your towel and prepare to pat down before you sweat it out in here!
Onyang Hot Spring (Tourist) Hotel
충남 아산시 온천1동 242
Chungnam-do, Asan-si, Oncheon 1-dong 242
041-540-2565Wikimapia of Onyang Hot Spring (Tourist) Hotel