A closer look
Before 1901, our understanding of the Earth’s interior was roughly 'it’s deep and probably hot.' Then Andrija Mohorovičić looked at seismograms from a Croatian earthquake and noticed that seismic waves were arriving at monitoring stations in two distinct phases. He realized the deeper waves were traveling much faster, proving they had bounced off a dense, solid layer beneath the crust. By tracking how shockwaves bend through rock, he discovered the Moho discontinuity, proving you can essentially give the planet an ultrasound if you just wait for it to break itself.
