Hosabes is a small mineralized spring lying in a gypsous crust in the central Namib Desert on the south-western coast of Africa. It is fed by virtually fresh groundwater that wells to the surface at this and other similar fracture lines. The extremely high rates of evaporation and solar radiation result in intense concentration of the salts and the formation of strong salinity and therefore density gradients in the deeper parts of the system. These in turn produce a 'solar pond' effect with a reversed temperature gradient; temperatures reach 50°C within the water column. The deeper parts of the system appear to be monomictic. Although the surrounding crust is largely gypsous, the water in the spring is dominated by NaCl as a result of precipitation of CaS04 in the highly concentrated water.