Using an array of 556 seismic sensors, Levander and Miller charted two key features of the subsurface structure of the western continental United States, with implications for explaining the locations of seismic and volcanic activity. The solid crust and the solid mantle of the Earth combine to form the lithosphere; together they overlay the plastic mantle of the asthenosphere. Deeper still are the upper and lower mantles and the liquid and solid cores. The boundary between the crust and the solid mantle is known as the Mohorovičić discontinuity (Moho), and the one between the solid and plastic mantle is the lithosphere-asthenosphere boundary (LAB). Measuring seismic waves produced by 163 earthquakes from 2005 to 2009, the authors charted the depth of Moho and LAB in the western United States and thus the thickness of the solid lithospheric mantle.