The Tool Desk
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What was the Kola Superdeep Borehole built to do?
The Kola project was a scientific effort to investigate the continental crust—not an attempt to drill to Earth’s center. Drilling of the SG-3 borehole began in 1970 on the Kola Peninsula in the former Soviet Union. Its original target was about 15 kilometers below the surface.
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The deepest point is commonly given as 12,262 meters. The German Research Foundation’s International Continental Scientific Drilling Program (ICDP) gives 12,261 meters, a one-meter discrepancy in published figures. Either way, the hole remained within the crust and did not reach the mantle.
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The ICDP describes two broad intervals in the borehole. From the surface to 6,842 meters, it identifies Lower Proterozoic sedimentary and volcanic rocks of the Pechenga Formation. Below that, down to its listed final depth of 12,261 meters, the borehole passed through Archean granitic and metamorphic basement, including gneisses, amphibolites, migmatites and granitoids.
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One expectation was that drilling would encounter a major crustal transition—the Conrad discontinuity, often associated with a change from granite-rich upper crust to denser, basaltic rock below. Instead, granite continued deeper than anticipated. The result complicated a simple picture of sharply divided crustal layers; it did not overturn geology as a whole.
What else did scientists find deep underground?
Mineralized fluids in fractured rock
Accounts of the borehole describe mineralized fluids circulating through deep fractures. A 2026 review also reports hydrogen-rich fluids. These findings concern fluids in deep rock, not a vast underground ocean.
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Fossil plankton, not living creatures
Researchers reported microscopic fossil plankton at depth. Smithsonian Magazine describes finds around four miles down, while a 2026 review places reworked fossil plankton at about six kilometers. These are reported fossil or reworked materials—not evidence of living organisms at the bottom of the hole.
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Why did drilling stop before 15 kilometers?
Conditions at the bottom were much hotter than expected, and heat was only one part of the problem. Difficult rock and borehole conditions, equipment malfunctions and shrinking funds combined to make continued drilling increasingly difficult.
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| Account | Bottom-hole temperature | Comparison or context |
|---|---|---|
| Smithsonian Magazine (2015) | 356°F (about 180°C) | Compared with a mapped expectation of 212°F (100°C). |
| Wrigley, in Environment and Planning D: Society and Space (2023) | Close to 200°C | About 120°C was anticipated at the planned 15-kilometer depth. |
| Communications Earth & Environment review (2026) | 212°C | Reports this as the bottom-hole temperature and describes rock flowing plastically and repeatedly sealing the hole. |
These accounts report different temperature figures; they should not be treated as interchangeable readings. Together, they describe a drilling environment that proved substantially more difficult than planners had expected.
The project’s end also depends on which milestone is meant. Smithsonian Magazine gives 1992 as the year the hole was abandoned. Wrigley’s 2023 account says the main shaft stopped in 1990, supplementary channels continued until 1994, and the project was mothballed in 2007. Those dates refer to distinct stages rather than one universally applicable shutdown date.
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How deep was it compared with its goal?
| Measure | Depth |
|---|---|
| Original target | About 15 kilometers |
| Commonly reported maximum | 12,262 meters |
| ICDP listed maximum | 12,261 meters |
The achieved depth was extraordinary for scientific drilling, but it was not close to the mantle. The project’s discoveries came from studying crustal rocks and conditions at depth, rather than penetrating into Earth’s deeper layers.
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