646 MOHO DISCONTINUITY enterprise, in that it attempted to resolve a geodynamic evolution from $3.5 billion years ago to the present The EGT project was followed by the EUROPROBE project, which aimed at analysing the structure, properties, and evolution of ‘deep Europe’ and exemplary cases of crustal accretion, thickening, and extension The summary of data from these projects covers the geophysical tools used, the petrological significance of the data, and the main structural features of the European Moho-discontinuity Geophysical Tools Geophysical methods are indispensable for the recognition and interpretation of structures in Earth’s crust and mantle Among the numerous geophysical methods suitable to explore structure and composition of the lithosphere, seismic techniques provide the best tools for detailed crustal and upper mantle investigations (see Seismic Surveys) In explosion seismology, the use of deeply, nearly vertical, reflected waves (P waves) in the subcritical range provides detailed information about the internal structure of the crust and the uppermost mantle (Figure 1A) Another method is based on recording P waves termed ‘PmP’ wide-angle arrivals; these are recorded as overcritical reflected waves from the Moho-discontinuity and/or as diving waves generated by a strong velocity gradient zone in the model of the crust/mantle transition (Figure 1B) Tomographic studies using travel times of waves generated by earthquakes or by big explosions allow determination of the velocity field of the lithosphere (Figure 1C) During the past decade, a new ‘receiver function method’ has been developed (Figure 1D) It is based on earthquake records of broadband stations and makes use of P-to-S converted phases that are created at the Moho-discontinuity or other prominent boundaries in the lithosphere For the study of the lithosphere/asthenosphere system, analysis of surface waves is another powerful seismic tool The data discussed here are mainly based on recordings of P waves (compressional waves) Nature and Stability of the Moho-Discontinuity From a seismological point of view, the Mohodiscontinuity is defined as the depth at which the velocity vP of compressional waves increases from normal crustal velocity values of 7.6 km s Modern studies see the Moho-discontinuity as a sandwichlike mix of crust and mantle materials (i.e., low- and high-velocity materials) In addition, the Mohodiscontinuity is not only a compositional boundary, Figure Basic principles of deep seismic sounding methods: (A) reflection seismology, (B) refraction seismology, (C) tomographic seismology, and (D) receiver function analysis