A Cold War race to the mantle
During the Cold War, competition between the United States and the Soviet Union extended beyond space and into the Earth itself. While American scientists pursued "Project Mohole" to drill through the thinner oceanic crust, Soviet researchers set their sights on the continental crust of the Kola Peninsula. Their goal was to penetrate the Mohorovičić discontinuity (Moho), the boundary separating the Earth's crust from the mantle. Drilling began on May 24, 1970, with a target depth of 15,000 meters.
The project, known as the Kola Superdeep Borehole, used a series of custom-built rigs like the Uralmash-4E and later the Uralmash-15000. Unlike drilling for oil, the mission was a purely scientific mission to sample the deep crust. The main borehole, named SG-3, eventually reached a true vertical depth of 12,262 meters (40,230 feet) in 1989, a record that still stands. This depth is greater than the deepest point of the ocean and surpasses the combined height of Mount Everest and Mount Fuji. The hole itself is surprisingly narrow, only about 23 centimeters (9 inches) in diameter at its widest point.
Unexpected geology and discoveries
The Kola project fundamentally changed geologists' understanding of the continental crust. Scientists had predicted, based on seismic data, a transition from granite to denser basalt rock at a depth of about 7 kilometers. This "Conrad discontinuity" was never found. Instead, the granite extended far deeper than expected, with the change in seismic waves attributed to metamorphic processes, intense heat and pressure altering the rock, rather than a change in rock type.
A surprising find was the presence of liquid water at depths between 3 and 6 kilometers. This water, trapped within rock crystals, was previously thought to be impossible at such depths. Another major discovery was biological activity in two-billion-year-old rocks. At a depth of 6.7 kilometers, scientists found 24 species of microscopic plankton fossils, remarkably preserved in organic compounds despite the extreme conditions.
Drilling finally stopped not because of equipment failure, but because of heat. The temperature gradient rose much faster than anticipated. At the bottom of the borehole, the temperature reached 180°C (356°F), nearly double the expected 100°C. At this temperature and pressure, the rock began to behave like a plastic, deforming and flowing into the borehole, making further progress impossible. The project was officially stopped in 1992, and the site was abandoned after the dissolution of the Soviet Union led to a loss of funding. Today, the deepest artificial point on Earth is sealed under a heavy, bolted metal cap.