Seismic P-waves from nuclear explosions at the Kazakhstan test site in Eastern Kazakhstan (KTS) observed at the Yellowknife-array (YKA, Canada) are used to analyze the structure at the base of the mantle under Severnaya Zemlya (Arctic Sea). 18 events at the Degelen subsite and 9 events at the Shagan River subsite of KTS are studied. The simple waveforms and well controlled source parameters of nuclear explosions allow the use of the events as source arrays in addition to the usual receiver array configuration. A new method (double-beamforming) integrating both concepts is presented. This method increases the resolution considerably. For the Degelen?YKA path one anomalous arrival (1.9 s before PcP, slowness between P and PcP slowness) is observed. Smaller anomalous arrivals are found 6.5 s (Degelen?YKA) and 8.1 s before PcP (Shagan River?YKA). These anomalous phases cannot be explained with standard Earth models. They are produced by reflections from an inhomogeneity in the lowermost mantle at (82.3N, 107.5E) under the area north of Severnaya Zemlya. The strong variations between the wavefields for the two source-receiver combinations indicate a heterogeneous structure in the lower mantle. The anomaly found here is located close to an anomaly in the lowermost mantle beneath the Nansen Basin. ¿ American Geophysical Union 1993