These are underground waters that naturally emerge at the surface with a temperature higher than the annual average air temperature of the location where they spring, generally above 20°C. They are often enriched with dissolved elements and gases that give them therapeutic properties. Thermal waters have been widely used for balneological purposes since ancient times. Italy is very rich in thermal waters and has a deep tradition in balneology dating back to ancient Roman times.
Globally, there are tens of thousands of thermal springs, with a significant concentration along the Italian Tyrrhenian margin, where numerous active and inactive volcanic districts are located.