A profile of deep seismic sounding across the South Caspian depression is described. The peculiarity of the structure of the depression is the reduced thickness of the Paleozoic and Mesozoic folded complexes, which is indicative of its geanticlinal development at that time. These complexes have seismic velocities in the range usual for granites. They were not discovered in earlier seismic studies in the Black Sea and Caspian Sea because the complexes are too thin, and the methods used did not allow discovery of detail. One peculiarity of these depressions is the so-called 'sedimentary-basaltic' crust. This arises when strongly metamorphosed rocks have been overlain by thick, poorly consolidated sediment, after a long period of submergence.