“Hot water in the City of the Thousand Fountains,” Aix-en-Provence, France, ca. 1910, by Guittard (published by C. Martinet), via Casas-Rodríguez Collection, under CC license.
The Fontaine d’Eau Chaude was the first fountain built on the Cours Mirabeau — in the 1600s. The hot water (34°C/64°F) comes from the hot springs of Bagniers. There are current photos here and here, showing it still covered in moss.