An earthquake's echo
In Zagreb on October 8, 1909, a powerful earthquake shook the Kupa Valley, 39 kilometers to the southeast. For Croatian seismologist Andrija Mohorovičić, this event provided the raw data for a fundamental discovery about the planet's structure. While analyzing seismograms from various monitoring stations, he noticed a curiosity: seismographs recorded two distinct sets of P-waves and S-waves, the primary and secondary waves generated by an earthquake.
Mohorovičić theorized that one set of waves traveled directly through the surface layer, while the other, arriving sooner at more distant stations, must have traveled down into a deeper, denser layer where waves propagate more quickly before refracting back to the surface. His calculations showed a sharp increase in seismic wave velocity at a specific depth. This boundary is now called the Mohorovičić Discontinuity, or "the Moho" for short. He determined that the crust's thickness was about 54 kilometers in the region he was studying. Modern science shows the Moho's depth varies, from as little as 5-10 kilometers under the oceans to 20-90 kilometers beneath continents. At this boundary, the velocity of P-waves jumps from around 6.7–7.2 km/s in the crust to 7.6–8.6 km/s in the mantle.
The quest to pierce the Moho
Directly sampling the mantle requires drilling through the entire crust. The first major attempt was the ambitious American Project Mohole in the early 1960s. The plan was to drill through the thinner oceanic crust off Guadalupe Island, Mexico. In 1961, the team successfully drilled five holes and reached a depth of 183 meters below the seafloor, penetrating into the basaltic crust. The project pioneered dynamic positioning technology to keep the drilling ship stable in deep water, but it was ultimately defunded by the U.S. Congress in 1966 due to mismanagement and rising costs.
The Soviet Union took a different approach with the Kola Superdeep Borehole, starting in 1970 on the Kola Peninsula. Instead of aiming for the thin oceanic crust, they drilled deep into the continental crust. Over two decades, the project reached an astonishing true vertical depth of 12,262 meters (40,230 feet), making it the deepest artificial point on Earth. Even so, it only penetrated about a third of the way through the thick Baltic continental crust. Drilling stopped in the early 1990s when temperatures reached an unexpected 180°C (356°F), causing the rock to behave like plastic and making further progress impossible. More recent efforts by the International Ocean Discovery Program (IODP) have successfully recovered mantle rocks from underwater mountains where tectonic activity has pushed them closer to the surface, but a complete drilling profile through the crust and the Moho remains an objective.