Baden is famous for its hot sulfuric springs that rise through cracks in the rock from hundreds of meters below the earth's surface. Hot spring water emerges in Baden in numerous places but only 14 springs are "curbed". That means they are walled and sealed so the water is pure and bubbles out forcefully:
Engelsquelle
(Angel's Spring) (32.9°C)
Ferdinandsquelle
(Ferdinand's Spring) (24.6°C)
Franzensquelle
(Francis's Spring) (30.4°C)
Frauenquelle
(Women's Spring) (34.5°C)
Johannesquelle
(Johan's Spring) (26.2°C)
Josefsquelle
(Joseph's Spring) (33.9°C)
Karolinenquelle
(Caroline's Spring) (34.6°C)
Leopoldsquelle
(Leopold's Spring) (27.8°C)
Mariazellerquelle
(Mariazell Spring) (24.5°C)
Marienquelle
(Mary's Spring) (34.9°C)
Peregriniquelle
(Peregrini Spring) (21.6°C)
Petersquelle
(Peter's Spring) (32.1°C)
Römerquelle
(Roman Spring) (29.0°C)
Sauerhofquelle
(Sauerhof Spring) (25.7°C)
(The indicated temperatures are based on measurements taken in 1996).

Already used by the Romans, the Source Spring, also known as the Roman Spring, emerges in a cave directly under the orchestra pit in the Sommerarena.
The spring in the Frauenbad had a church directly on top of it until 1811, the old Frauenkirche, whose topsy-turvy tower was a landmark in Baden. The spring bubbles to the surface at a spot directly below the site where the high altar once stood. The water from the Peregrini spring was sold as mineral water well into the 1960s. Mary's Spring has the biggest output, its water issuing at a rate of more than 40 liters per second. It was impounded for the first time in 1924 and is the main source of sulfuric water for the Baden beach baths. Today all springs except the Angel's Spring are under the control of the city. Their water is channeled into a central closed circular pipeline. Treatments are administered exclusively in the sanatorium (Kurhaus) next to the Römertherme. It is thanks to these hot springs that Baden remains the most important tourist resort in Lower Austria to this very day.