At the end of the walkplatz in Wiesbaden....is Kochbrunnenplatz. It's a combination park and warm water spring. In the midst of it is a covered area over a hot water spring.
It sits adjacent to Taunus Strasse. Walking from the train station, it'd probably take you thirty minutes to reach the park.
If you went back a hundred years....this was a popular spot for vacationing VIPs (Russia, US, France, Germany, England, etc)....to do a stroll on a summer afternoon, and use the waters for medicinal purposes.
Today? You might occasionally see someone stop by and douse themselves with a bit of water. There's a sign on the interior of the structure which details the contents of the water.....to finite detail. Germans are that way....if you didn't know.
On the list of twenty-odd things in Wiesbaden worth checking out.....I'd put it between ten and twenty. You don't come across things too often like this in life.