Deeper than the Mariana Trench, near its border with Norway, the Soviet Union spent 20 years drilling 7.6 miles into Earth; in 1992, the deepest borehole on Earth was abandoned in a region industrialization had turned into one of the most ‘poisoned spots on Earth’

Earth has its own vertical world. Above us is the sky; below it are layers of rock accumulated over billions of years, holding fossils, minerals, heat and secrets that humans can reach only by cutting into the ground.

On the Kola Peninsula, near Russia's border with Norway, the Soviet Union tried to go farther down than anyone had gone before. In 1970, the Cold War was pushing the superpowers upward into space, but the mighty Soviets also wanted to conquer what lay beneath their feet. As the country celebrated the centenary of Vladimir Lenin's birth, engineers began drilling into the Earth's crust.

For almost 20 years, the drill descended through ancient rock. The result was the Kola Superdeep Borehole, a man-made crevasse that eventually reached 12,262 meters, or about 7.6 miles, beneath the surface, deeper than the Mariana Trench's Challenger Deep, which reaches about 10,994 meters (6.8 miles).


But the Soviet Union's achievement of drilling the deepest artificial hole in human history came with a lesson few could have anticipated: the deeper humans went, the less Earth behaved as engineers expected.

The Soviet race to reach Earth's depths

Drilling began on May 24, 1970, on the Kola Peninsula, a remote stretch of land that borders Norway and lies far above the Arctic Circle. At that time, the peninsula was part of the Soviet Union, which was once the domain of the indigenous semi-nomadic Sami people.

The Kola project was part of a much larger Cold War competition. The Soviet Union and the United States were already competing in space, nuclear technology and military power. Beneath the surface, scientists and engineers were also trying to understand how far humans could penetrate Earth's crust.

The ambition was extraordinary: drill deep enough to study the Earth's crust directly and, ultimately, approach the boundary between the crust and mantle, a solid layer of hot silicate rock that lies directly beneath it.

For almost two decades, the drill kept going. By the time the project ended in 1992, the Kola Superdeep Borehole had reached 12,262 meters. That was still only a fraction of the distance to Earth's core and, importantly, the drill had not reached even the mantle.

Yet the hole had already produced an unprecedented look at the planet's continental crust, rarely seen by any species.

What scientists found underground

The deeper the Soviet drill traveled, the older the rocks became. According to Orion Magazine, the borehole passed through deposits containing copper, nickel and magnetite before reaching rocks resembling samples recovered from the Moon. Eventually, scientists encountered rocks estimated at about 2.5 billion years old, containing ‘microscopic plankton fossils.’


But the greatest obstacle was not simply distance; it was heat. A 2026 scientific review published in Communications Earth & Environment found that temperatures at the bottom of the borehole reached about 212 degrees Celsius. At such depths, the rock behaved differently from what drilling engineers had anticipated, producing a “plastic rock flow that repeatedly sealed the hole.”

“The SG-3 well fundamentally reshaped understanding of continental crustal structure and deep geologic processes,” the researchers wrote. “Despite not penetrating the Moho [boundary between Earth’s crust and mantle], SG-3 yielded the most extensive direct dataset on the crystalline continental crust.”

The hole that outlasted the Soviet Union and open a new energy source

Physical, technological and financial constraints eventually brought drilling to an end in 1992, a few months after the Union disintegrated. The hole remained, however, as a record of how far humans had managed to reach into solid Earth.

Its importance has not entirely disappeared. Scientists are now looking again at the extreme heat beneath Earth's surface for a very different reason: geothermal energy.

Enhanced geothermal systems can use deep wells and high-pressure water to extract heat from hot rocks and bring that energy back to the surface. The deeper the drilling, the greater the potential access to high-temperature rock that could potentially be used for power generation.

Then Kola Superdeep Borehole region becomes a poisoned landscape

But the story of Kola cannot be separated from the landscape surrounding it. The borehole was located in a region already heavily shaped by mining and heavy industry. By the 1990s, the wider Kola Peninsula had become synonymous with severe industrial pollution.

A 1994 New York Times report described the region as one of the most “poisoned spots on Earth,” documenting extensive environmental damage around its mining and metal-processing centres. The report described forests damaged by industrial pollution, contaminated waterways and serious concerns about the health of people living and working in the region.

The contrast was striking. The same remote northern landscape that had become a showcase for Soviet scientific ambition was also carrying the consequences of decades of industrial activity.

The Kola Superdeep Borehole itself was eventually abandoned. What remains is less a visible monument than a hole sealed deep underground and a scientific record of a moment when the Soviet Union tried to push through Earth's crust simply to discover what was there.