Description: In March 1974 combined refractional and reflectional seismic experiments were performed in the Cretan Sea. A total of 141 shots were fired along two lines, one parallel to the main structure striking nearly E-W between Karpathos and the SE-Peloponnese, the other perpendicular to it between Crete and Santorin (Thira). The shots were observed at 13 sites on land and by four buoy stations at sea. The seismic data reduced to time-distance plots and evaluated by direct and inverse ray theory methods revealed that: The Cretan Sea builds an E-W elongated dome of the upper mantle with maximum crustal attenuation at its central part and a thickness of nearly 20km. The vertical distribution of the P wave velocity is that of a continental crust that has been subjected to attenuation. The present distribution of sediments is mainly confined to the zone: SE-Peloponnese-Kithera-Antikithera-Crete-Karpathos, and also to small depressions within the central basin. In general they do not exceed 2 km in thickness. The uneven distribution of the sediments indicates the possibility of a two phase subsidence of the Cretan Sea. Areas with no or very thin sedimentary cover belong to the youngest tectonic phase. The morphology of the crystalline basement has been formed by intensive fragmentation, consisting mainly of normal faults, typical for tensional tectonism. The northern border of the Cretan Sea exhibits intensive volcanism along the line : Methana-Milos-Santorin-los. This zone is partly mobilized and intruded by surplus masses from the upper mantle as shown in the case of Santorin.