Named after Croatian seismologist Andrija Mohorovičić, this boundary marks the transition to a solid, rocky shell comprising 84 percent of Earth's volume.

In 1909, researchers realized that seismic vibrations from a Croatian earthquake were arriving at distant monitoring stations faster than expected.

Sound and seismic energy travel faster through the tightly packed atoms of the dense peridotite rock found in the upper mantle.

The 1960s Project Mohole attempted this oceanic drilling because seafloor crust is only 5 to 10 kilometers thick compared to 70 kilometers on land.