The term thermal refers to heat. Thermal or geothermal water is a natural resource of warm water which temperature is above 30 C° (86 F°). Thermal water originates from the deeper layers of earth's solid mantle where temperature is higher than close to the surface. Descending groundwater is heated up by geothermal heat. The warm, thermal water is forced to the surface by the high pressure within the solid earth mantle and reaches the surface through natural channels, hollows, caves as hot springs or through artificially drilled wells.
Because of its high temperature thermal water can contain substantial quantities of dissolved solids from deeper layers of the earth mantle. The mineral content can be very different between geographic areas and can have beneficial health effects when used internally or externally. Thermal bathes, natural spas are renown for their relieving effects against various skin and rheumatic diseases, though a physician's advise is always necessary in such cases.