On a patch of tundra in Russia’s far northwest, close to the Norwegian border, there is a small, rusty metal cap. It is bolted down, and the number 12262 is written across it. That is how many meters the hole underneath goes down. It is the deepest hole people have ever drilled, a little over 12 kilometers, or about 7.6 miles, straight into the Earth. It is also only 23 centimeters (9 inches) wide. Soviet scientists spent about two decades drilling it, and what they found at the bottom surprised almost everyone.
A race in the other direction
In the late 1950s, while the United States and the Soviet Union were racing into space, a quieter contest was going on in the opposite direction. Geologists on both sides wanted to drill through the Earth’s crust, the thin rocky skin we live on, and reach the mantle underneath. The boundary between the two is called the Mohorovičić discontinuity, or the Moho for short, after the Croatian scientist who spotted it in earthquake waves in 1909.
The Americans went first. In 1957, a group of scientists proposed Project Mohole, a plan to drill through the seafloor, where the crust is thinnest. In 1961, a test drilling ship off Guadalupe Island, Mexico, bored about 183 meters (601 feet) into the ocean floor through 3,600 meters of water, and pulled up cores that ended in the volcanic basalt beneath the seafloor mud. The novelist John Steinbeck covered it for Life magazine. But the project’s costs ballooned, its managers fought, and in 1966 Congress cut off the money. The idea did not die completely. It led to decades of scientific ocean drilling that taught researchers a great deal about the seafloor.
The Soviets chose a different route. Instead of going through thin ocean crust, they would drill into very old, very hard rock on land, on the Kola Peninsula, well above the Arctic Circle. They started on May 24, 1970.
How do you drill 12 kilometers?
The first goal was 7,000 meters, using a slightly modified oil-drilling rig. In 1974, the team brought in a new, purpose-built rig called the Uralmash-15000, named for the new target: 15,000 meters. On June 6, 1979, Kola beat the world depth record of 9,583 meters, held until then by a well in Oklahoma.
Drilling that deep creates a basic problem. A normal oil rig turns the whole string of pipe from the top, and the bit at the bottom spins with it. At Kola, that pipe would have weighed more than a million pounds and stretched for miles, so twisting all of it was not practical. The Soviet engineers instead put a turbine at the bottom of the hole. Drilling mud pumped down the pipe under pressure spun the turbine and the bit, while the pipe itself mostly just slid deeper. Changing a worn bit still meant hauling the whole string back up to the surface and lowering it again.
Things went wrong regularly. The hole was drilled as several branches, with new ones started partway down when an old one became unusable. After the drill passed 12,000 meters in 1983, work stopped for about a year for visits and celebrations. Not long after it started again, in September 1984, a section of drill string about 5 kilometers long twisted off and was lost down the hole. The crew had to back up to 7,000 meters and drill a new branch from there.
In 1989, that new branch reached 12,262 meters, or 40,230 feet. Its bottom is deeper below the surface than the deepest point of the ocean is below the waves, and it is more than 3 kilometers deeper than Mount Everest is tall.
What they found down there
The point of the project was not just to set a record. The scientists wanted to see what the deep crust is really made of, instead of guessing from earthquake waves. That is where the surprises came in.
There was no basalt. Seismic readings had shown a sharp change at about 7 kilometers down. Many geologists expected that to be the place where granite gives way to heavier basalt rock. The drill found no basalt there, or at any depth. The granite simply kept going. The change that showed up in the seismic data turned out to be a shift within the granite itself, from one form to another that had been reshaped by heat and pressure.
There was water. At that same zone, the rock was badly cracked and full of hot, mineral-rich water, far deeper than most scientists expected to find free water. The water’s chemistry suggested it had not trickled down from the surface. Instead it seems to have formed from the rock’s own minerals and then been trapped beneath a layer of rock it could not pass through.
The mud seemed to boil. Drilling mud coming back up the hole was described as “boiling” with hydrogen gas. Helium, nitrogen, and carbon dioxide came up too.
There were fossils. At about 6.7 kilometers down, in rock roughly two billion years old, researchers identified 24 species of microscopic fossil plankton. They were wrapped in carbon and nitrogen compounds that seem to have protected them from crushing pressure and heat. At the very bottom, the rock was about 2.7 billion years old.
It was much hotter than planned. The team had expected about 100°C (212°F) at the bottom. They got about 180°C (356°F). At that temperature and pressure, the rock started to behave less like stone and more like very thick putty, slowly creeping back into the hole. The drilling tools could not cope.
Why it stopped
The crews kept trying for a few more years to push past 12,262 meters, but they never got deeper than the 1989 mark. In 1992, a newer branch was stopped at about 11,900 meters by the heat. The Soviet Union had collapsed, the money ran out, and a last attempt was abandoned in 1994. By 2008, the site was completely abandoned. The buildings have since been stripped and left to fall apart, and the hole was capped and welded shut.
Even so, the hole was only about a third of the way through the crust in that part of the world, which is roughly 35 kilometers thick there. Measured against the whole planet, it got about 0.2 percent of the way to the center.
The “Well to Hell” that wasn’t
A hole this deep was bound to attract legends. Around 1989, a story started moving through Christian newsletters in Finland and then the United States. It claimed that Soviet engineers in Siberia had drilled 14.4 kilometers down, broken into a huge cavity, measured a temperature of 1,000°C, and lowered a microphone that picked up the sound of screaming. The tale was broadcast on American religious television as proof that hell is a real place, and recordings of the “screams” are still passed around online.
None of it holds up. The real hole was on the Kola Peninsula, not in Siberia. It was never 14.4 kilometers deep, and nobody reported anything supernatural. The famous recording has been traced to sound effects, often identified as coming from a 1972 horror film called Baron Blood. A Norwegian teacher who heard the story on TV even added a ridiculous twist, a bat-like creature flying out of the hole, to see whether the network would check its facts. It did not, and it aired that version too.
Others have tried since
Germany ran its own superdeep project, known as KTB, in Bavaria from 1987 to 1995. Its main hole reached 9,101 meters in October 1994, where temperatures topped 260°C (500°F). As at Kola, the rock was far less solid and dry than expected, with fluids and gas pouring into the hole. The giant drilling tower still stands there, next to a visitor center.
Some oil wells are now longer than the Kola hole, but they are not deeper. In 2008, a well at the Al Shaheen oil field off Qatar stretched 12,289 meters in total, but most of that runs sideways, and its deepest point is only about 1,400 meters down.
The closest challenger is in China. A well called Shenditake 1, in the Taklimakan Desert, reached 10,910 meters in January 2025, which its operator says makes it the deepest vertical well in Asia and the second deepest in the world. Drilling the first 10,000 meters took less than 300 days. The last 910 meters took more than 300 more, which shows how hard every extra meter gets that far down.
The heat that stopped the Kola drillers is also exactly what some energy companies now go looking for. Geothermal developers drill into hot rock to make steady electricity, as we covered in our piece on Fervo’s hot-rock power plant in Utah.
The bottom line
The Kola Superdeep Borehole never reached the mantle, and it barely scratched the crust. But it showed that the inside of a continent is wetter, hotter, gassier, and more surprising than the textbooks of the 1970s predicted. More than 35 years later, its record still stands, under a bolted cap on the tundra.
Crack open some rock yourself, or read about the world underground
National Geographic Break Open 10 Premium Geodes — Ten real geodes, safety goggles, display stands, and a learning guide. Splitting one open is a small, hands-on way to see what rock hides inside, no 12-kilometer drill required.
Underland: A Deep Time Journey — Robert Macfarlane travels through caves, mines, ice, and a deep tunnel built to store nuclear waste. A beautifully written trip into the world beneath our feet.