Says Wikipedia: The Széchenyi Medicinal Bath in Budapest (Széchenyi-gyógyfürdő) is the largest medicinal bath in Europe. Its water is supplied by two thermal springs, their temperature is 74°F/23°C and 77°F/25°C, respectively.
The bath can be found in the City Park, and was built in 1913 in Neo-baroque style to the design of Győző Czigler.
Go there! It's quite an experience, especially if you arrive in the wintertime to swim in the outdoor pools, (there are plenty of indoor options available).