Caltech researchers have developed a new method to study Earth's structure deep beneath the surface, at the boundary between Earth's brittle crust and the underlying mantle, a region called the Mohorovičić discontinuity—Moho for short. Like taking an ultrasound, the method measures how seismic waves from earthquakes are reflected off of the Moho and utilizes cutting-edge seismic technology called distributed acoustic sensing (DAS).