No. 67: Jan-Feb 1990
From the study of seismic waves, geophysicists have determined that between 7.5 and 8.6 kilometers below the surface there exists a clear-cut "discontinuity." Practically speaking, this means that above this layer seismic waves travel at a markedly different velocity than they do below it. This discontinuity is so widespread, occurring beneath all of the continents, that it has received a special name: the Conrad Discontinuity.
Ordinarily, a geophysicist would expect to find a significant change in rock type when drilling through such a strong discontinuity. It was widely expected that, at the Conrad Discontinuity, drillers would find the granitic rocks typical of the continents changing suddenly into basalt, which is thought to make up the lower reaches of the earth's crust. However, when Soviet drills pierced the Conrad Discontinuity below the Kola Peninsula, they found no such switchover to basalt at all. In fact, they hadn't even found it when they penetrated to 12 kilometers.
This was a shocker. Now, no one knows what the Conrad Discontinuity represents. It doesn't signal a change in rock type; neither is there a fault or boundary of any kind. It is important to find out what is wrong here, because much of modeling of the unseen structure of the earth's crust depends upon a realistic interpretation of seismic records. (Monastersky, Richard; "Inner Space," Science News, 136:266, 1989.)
Reference. Large-scale structural anomalies of the earth's interior are classified under ECD in the catalog: Inner Earth. This book is described here.