The upper layer of Lake Pavin has the usual physicochemical characteristics of a temperate zone freshwater lake. Its deep stratum, however, is an overpressurized, hostile and cold environment, devoid of light and oxygen and deserted by fish and zooplankton. Only microorganisms are present in this zone, rich in bacteria and reminiscent of oceanic pits. Although they do not mix, the two layers of the lake are not isolated from each other but exert close biological and chemical interactions.