It differs fundamentally from the other lakes in its physical-geographical aspects, even though its genesis is the same (fluvio-marine boundary). The average water temperature is generally close to that of the air. Due to its high mineralisation, the water temperature of Lake Techirghiol can drop below 0°C without freezing. The invertebrate fauna inhabiting Lake Techirghiol is dominated by the crustacean Artemia salina, which together with the alga Cladophora cristalina provides the raw material for the production of sapropelic mud with active mineral components that give it particular therapeutic value.
The sludge is found in sediment form on the bottom of the lake, in certain areas of the lake, over a total area of about 98 ha, in a layer about 0.6 m thick. Silt is a chemically complex substance formed by a process. Black, shiny and unctuous, it has a characteristic odour (hydrogen sulphide) and physical properties that make it extremely useful in the treatment of various ailments; it is unique in every respect. Mud has the property of spreading and moulding to the surface of the body (it is plastic); it absorbs water like a sponge and is also able to mix very well with it (it is hydropexic); it absorbs heat easily (it is thermopexic) when heated and gives it off with difficulty to our body (it has a low thermoconductivity); therefore, when applied hot to the skin, it feels less hot than a bath with water at the same temperature.