Large samples of teleseismic P-wave data recorded from Shagan River under-ground explosions have been systematically analyzed in an attempt to understand the causes of mb variability observed for these explosions. Results of these analyses indicate that large (>0.5 magnitude unit) differences in station-corrected mb residuals between explosions in close proximity are associated with changes in the near-source P-wave propagation paths to teleseismic distances. Back projection of the mb residuals from seismic stations in continental Europe into the P-wave initiation area near the source reveals the existence of an anomalous volume of material located about 50 km northwest of the test site, apparently at a depth of about 100 km. This anomalous volume defocuses energy out of paths to certain stations and redirects this energy into the paths to other stations and is associated with no detectable travel-time anomaly. This anomalous volume most likely corresponds to some relatively sharp lateral variation in physical properties, in particular P-wave velocity, at this depth. The estimated lateral dimension of the anomaly is only on the order of 5 km, which would make it one of the smallest deterministic features ever isolated at this depth range.