Most schoolchildren learn that the Earth has three (or four) layers: a crust, mantle and core, which is sometimes subdivided into an inner and outer core. That’s not wrong, but it does leave out several other layers that scientists have identified within the Earth, including the transition zone within the mantle.

In a study published this week in Science, Princeton geophysicists Jessica Irving and Wenbo Wu, in collaboration with Sidao Ni from the Institute of Geodesy and Geophysics in China, used data from an enormous earthquake in Bolivia to find mountains and other topography on the base of the transition zone, a layer 660 kilometers (410 miles) straight down that separates the upper and lower mantle. (Lacking a formal name for this layer, the researchers simply call it “the 660-km boundary.”)

https://www.princeton.edu/news/2019/02/14/massive-1994-bolivian-earthquake-reveals-mountains-660-kilometers-below-our-feet

Massive 1994 Bolivian earthquake reveals mountains 660 kilometers below our feet

Princeton seismologists found mountains and smooth plains on a boundary layer inside the Earth’s mantle.

^{Princeton University}