Water that comes to the surface from deep within the earth. They are characterised by high temperatures and a particular chemical composition. They are used for the organisation of industrial fishing. There are conditions for using highly mineralised geothermal waters for mariculture. Groundwater heated by the earth's heat has a special chemical composition. They are used for industrial fish farming.

The surface emergence of groundwater heated above +20°C. There is also a definition that a spring is a hot spring if it has a temperature higher than the average annual temperature of the area. 

Most hot springs are fed by water heated by magmatic intrusions in areas of active volcanism. However, not all thermal springs are tied to such areas; the water can also be heated by convective circulation - groundwater seeping down to a depth of about a kilometre or more, where the rock has a higher temperature due to the geothermal gradient of the Earth's crust, which is about 30°C per kilometre for the first 10 km. 

Thermal mineral springs are classified as warm (+20...+37°C), hot (+37...+50°C) and very hot (+50...+100°C).