The thermal water originates in the source region of Florentine Hill in Baden-Baden. There are a total of twelve individual sodium chloride springs originating from a depth of around 2,000 metres. The artesian pressure of the earth’s mass forces the water to the surface at temperatures between 56 and 68.8 °C, making them the hottest springs with the highest mineral content throughout Baden-Württemberg. The springs are around 12 to 17 thousand years old and expel approximately 800,000 litres of thermal water every day.
The hot springs formed in diluvium long ago and, since this time, a sinter hill of black, porous limestone has been growing below the springs. Some of this material was cleared away for the construction of Friedrichsbad and the stones were used, for instance, to build the Fettquelle spring structure and the fountains in Lichtentaler Allee.
The black limestone is also partially responsible for the fact that Florentine Hill is warmed through by the sun from early spring to late autumn, which is why some plants not native to the region tend to flourish here.