From Greek σαπρός "rotten" + πηλός "clay; silt, mud".

Sapropel - muddy bottom sediments of organic origin from eutrophic waters and marine sediments formed in the absence of oxygen from soil with high organic matter content.

Centuries-old bottom sediments of freshwater bodies, formed by dead aquatic vegetation, remains of living organisms, plankton, as well as particles of soil humus, containing a large amount of organic matter, humus: lignin-humus complex, carbohydrates, bitumen and others in colloidal state.

If the organic content is lower, the sediments are referred to as mineral muds. Sapropel is a gelatinous or granular mass of pink to brownish-olive and almost black colour. It hardens as it dries and cannot be soaked.

Sapropel is formed under the influence of biochemical, microbiological and mechanical processes. The formed sapropel is a complex organomineral substance complex. The composition of the organic matter of sapropel includes, in addition to the remains of plants and animals and the products of their decomposition, the products of microorganisms.