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From 1968 until 1990, Russian scientists carried out an intensive program of deep seismic sounding across the territory of the former Soviet Union, using Peaceful Nuclear Explosions (PNEs) as powerful sources for elastic waves. The explosions, both chemical and nuclear, were recorded by up to 400 shot-period (1-2 Hz) three-component, analog recording systems. The average station spacing of about 10 km along the profiles provided a data density not previously available for studies of the upper mantle and the transition zone. Observation distances of more than 3000 km allow the investigation of the velocity structure of the Earth's crust and upper mantle to a depth of 700 km. The analog data have been digitized and used to constrain the fine structure of the upper mantle below Northern Eurasia. They reveal reflections and refractions from upper mantle discontinuities at 410, 520 and 660 km depth. Several properties of the recorded phases have been used to derive a regional P wave velocity model. Synthetic seismograms were calculated and compared with the observations to test these models. Characteristic for all the data in northern Eurasia is the absence of strong pre-critical reflections predicted by the global IASP91 model for the 660 km discontinuity. The appearance of two additional characteristic travel-time branches in the distance range of 2200 km was interpreted as being caused by the proposed and disputed upper mantle discontinuity at 520 km depth. Synthetic seismograms were calculated to constrain its properties. The recordings on the Quartz profile in Northern Eurasia have been used to constrain the nature of the globally observed high-frequency teleseismic P$_{\rm n}$ phase, which can be observed for shot-receiver distances of more than 3000 km. We suggest that this phase is caused by velocity fluctuations in the upper mantle acting as scatterers. This hypothesis was tested by extensive numerical simulations of the wave propagation using finite difference methods. The present paper is an overview of an extended cooperative study of the upper mantle carried out with PNE seismic data. Although many studies have been carried out in the past, several major topics, for instance the S-wave structure, distribution of attenuation and anisotropy, need further investigations. [ABSTRACT FROM AUTHOR]