In the eastern Mediterranean, numerous argilo-kinetic manifestations, commonly named 'mud volcanoes', have been identified and studied in some detail during the last twenty years using several techniques. The Medinaut survey (December 1998) has provided new insights into this phenomenon through twenty deep dives performed with the submersible Nautile within two areas particularly active in terms of mud expulsions: the central Mediterranean ridge south of Crete, and the Anaximander mountains area, south of Turkey. The morphology, petrography, microstructure and biostratigraphy of different clasts sampled in the two areas helped to better understand the mud volcano formation and subsequent evolution. Paleoenvironmental conditions, ranging from deep marine deposition in distal deep-sca-fan environment during the Miocene (Mediterranean ridge) to continental margin conditions (Anaximander mountains), have been evaluated for the composition and age of the various rock types. Macroscopic and microscopic structural observations have revealed a close control of clast shapes by sets of early microfractures (calcite veins, joints) created before the clast expulsion to the seabed surface. Surface features, such as erosion marks, alteration aureoles or calcitic coatings, have also been described on most samples. Finally, erosion traces, alteration aureoles and calcitic coating observed for the first time are interpreted as the result of a complex cold seep context related to mud expulsion and associated fluid venting. (C) 2001 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS.