Tomographic inversion of Russian peaceful nuclear explosion (PNE) data collected along the 3500 km east-west striking profile Kraton has provided new information about the two-dimensional P-wave velocity structure of the upper mantle beneath the Siberian platform. The 8¿ discontinuity is clearly identified as the top of a zone in the 100--200 km depth range. This zone appears as a low-velocity zone in large parts of the profile and has zero velocity gradient along the profile. The 400 km discontinuity is imaged as an increase in velocity of at least 0.4 km/s at ~380--420 km depth. ¿ 1999 American Geophysical Union