Mapping Earth's deepest secrets

Sequencing seismograms pinpoint new structures near Earth’s core-mantle boundary

Deep within Earth's interior, at ∼2900 km beneath the surface, lies the boundary between the solid silicate rock mantle and the liquid iron-nickel alloy core (the core-mantle boundary). Geophysicists have studied the complex thermal and chemical dynamics that take place in this boundary layer. In the early 20th century, Gutenberg investigated the structure of the lowermost region, or base, of the mantle by recording with only a few seismograms from a small number of large-magnitude earthquakes that occurred thousands of kilometers away (1). The structure of the rocks just above the core-mantle boundary—designated as D″ by Jeffreys in 1939 (2)—forms a distinct layer with surprising complexity. Now, on page 1223 of this issue, Kim et al. (3) describe new structural heterogeneities in the lowermost mantle with the use of a learning algorithm that does not require any a priori knowledge of Earth.