Groundwater is said to be 15 000 years. This water contains various minerals, sulfates and calcium chloride radioactive, thermal and hyperthermal magnesium and iron. Some sources are hot - up to 51^o.

The water used by the Thermal Baths for pain relief. The mineral elements have several effects: calcium has a calming effect, anti-inflammatory and supports the development of bones.  Magnesium has a central calming effect. The therapeutic effect of thermal water results in a reduction in painful symptoms in degenerative and inflammatory rheumatic diseases.

In total there are 16 different sources in and around Rennes les Bains.