Unique properties acquired over the course of a millennium

Millennial waters

The complex and passionate formation process of the thermal waters involves an intense circuit from the entrance of the waters towards the depths of the earth, until it wells up again at the surface, which takes place over the course of thousands of years and kilometres. In this sense, the waters at Caldes de Boí are the oldest in the Pyrenees: over 16,000 years old.

The distinction between the waters

“Thermal” is the word used to describe the subterranean waters that have a temperature 5ºC higher than the average temperature of the upper aquifers in the area. And mineral waters are those that contain an unusually high concentration of certain dissolved chemical substances. When there is a combination of these two characteristics – temperature and mineral concentration – we have thermo-mineral waters. When their healing properties are officially recognized, they are named minero-medicinal waters. The transformation that these waters undergo to gain these properties and classification is a hydrogeological process with common elements, but every aquifer has its own history. The speed of the water flow, plus the level of contact with the materials and the characteristics of those materials are also key factors that contribute to the resulting chemical composition and properties of the waters.

Glacial waters and meteoric waters, or rain water, are the starting point of the hydrogeological process. These waters, which have special chemical characteristics due to the simple fact of their origin, follow an intense subterranean course through impermeable rocks and geological materials in the earth’s crust and the different layers of the lithosphere, which add chemical and thermal characteristics until they become thermo-mineral waters (because of their temperature and chemical composition). In the case of Caldes de Boí, the waters have now been officially recognized for many years as minero-medicinal due to their exceptional properties.


The waters that flow from the 37 sources or springs in Caldes, each one with different properties which are currently utilized in the spa, have taken 16,000 years to complete this journey to the bowels of the Earth and back to the surface, converted into sulphurous, ferruginous or carbonated waters, depending on the chemical composition, with different temperatures that range from 4ºC to 56ºC.

The exceptionality of Caldes de Boí also lies precisely in this variety of emanations, which were worthy of an entry in the Guinness Book of World Records.

Minero-medicinal waters can never be compared to tap water as they have special mineral properties, their origin is unique, they are pure, they don’t contain any kind of chlorine or disinfectant and they are certified as such.