Here's the story. In May 1992, a nuclear device was detonated in China. This underground test generated a one-second seismic punch that penetrated thousands of miles into the earth. Nearly on the other side of the globe, chains of seismometers in Canada and the United States picked up the pulse. Light doesn't penetrate into solid earth, so it's necessary to use some other sort of waves to provide images of what's within.
Seismic waves are the best researchers have been able to use so far.
They now have what amounts to a snapshot of a portion of the boundary between Earth's molten metallic core and the overlying mantle. The region that their wave traces delineated lay in the western Arctic, roughly from the area under Thule in northern Greenland to below Wrangell Island. The lower mantle in most of this region seemed to be homogeneous, but not in the area near Point Barrow. According to John E. Vidale and Harley M. Bent of the U.S. Geological Survey, a kind of subsurface island lies at the core-mantle juncture 2700 kilometers straight down under northern Alaska. That sounds like a rather large lump, but it's the smallest anomaly anyone yet has been able to identify in that deep and virtually invisible realm.