The thermal springs are on the south-east slope of the Florentinerberg. The hot water (up to 60°C) rises up from a depth of around 2000 metres to exit via several springs. The springs are the hottest and have the highest mineral levels of any springs in Baden-Württemberg. The water, which is slightly salty to taste, contains high levels of sodium and chloride.
In historical times, the original spring in the basement of the Old Steam Bath with a daily output of 133 cubic metres was the most important and most abundant main spring in the thermal region.
When the bathing region was redeveloped and the Friedrichsbad spa rebuilt, two adjoining tunnel systems were built in 1868 – 1871 to collect the main thermal water exits and increase the output. Following expansions, which were carried out around the turn of the 19th/20th centuries, the overall length of the tunnels is today in the region of 200 metres. They are brick-lined and accessible. The daily output from all the springs is more than 800 cubic metres.
The thermal water was used for bathing and drinking cures. It has been used for bathing since Roman times. From the 16th century onwards and until just a few years ago, the hot water was also used for well cures. One of the main therapeutic benefits of taking the salty mineral water internally was on the mucous membranes of the digestive tract.
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