How deep could humans dig if they really tried? During the Cold War, scientists in the Soviet Union set out to find out. They picked a spot in the far north, near the border with Norway, and started drilling straight down. They kept going for almost two decades.
The result is the Kola Superdeep Borehole, the deepest human-made hole on Earth. In 1989 it reached a depth of 12,262 meters, about 40,230 feet or more than 7.5 miles. Yet the hole itself is only about 9 inches across. Even at that depth, it went only about a third of the way through the continental crust in that region.
A Race Into the Rock
Drilling began on May 24, 1970, on the Kola Peninsula. The original goal was 7,000 meters, but the target was soon raised to 15,000 meters, nearly 50,000 feet, and a new, purpose-built drilling rig was brought in.
On June 6, 1979, the project broke the world depth record, which had been held by a hole in Washita County, Oklahoma, at about 31,440 feet. Rather than one hole, the crews drilled several, branching off from one another as they went.
It wasn't smooth going. After the drill passed 12,000 meters in 1983, work paused for about a year for scientific and celebratory visits, and that idle time may have contributed to trouble later. In 1984, after reaching about 39,587 feet, a section of drill pipe roughly 3 miles long twisted off and was lost in the hole. The crews started again from a shallower point and pushed on to the record depth in 1989.
Too Hot to Keep Going
The deeper they drilled, the hotter it got. In 1992, work on one of the holes stopped because temperatures reached 356 degrees Fahrenheit, much higher than expected. A final attempt ended in 1994 for lack of money, and drilling was officially over by 1995.
The site was eventually abandoned in 2008. Sightseers still visit and report that the structure over the borehole has been partly destroyed or removed.
What They Found Down There
The project was a scientific one, and the rock had surprises. Geologists expected to hit a layer of basalt about 4.3 miles down. They never found it at any depth. Instead they found more granite than predicted.
They also found water 2 to 4 miles below the surface, trapped beneath a layer of rock it couldn't pass through. The drilling mud coming out of the hole was described as "boiling" with hydrogen gas. Most remarkable of all, microscopic plankton fossils turned up about 3.7 miles below the surface.
The Legend of the Well to Hell
The borehole also inspired a famous hoax. A story spread that Soviet scientists had drilled into hell and recorded the screams of the damned. It's an urban legend. The scientists reported geological surprises, not supernatural ones, and the so-called recording was later found to be stitched together from sound effects, sometimes identified as a 1972 horror movie soundtrack.
Other countries have tried deep drilling too. The United States started Project Mohole in 1957 to drill through the thin crust under the Pacific, but it was abandoned in 1966. Germany's KTB borehole reached about 29,859 feet, and in 2023 China began a scientific super-deep borehole in the Tarim Basin.
For now, the Kola hole still holds the crown. More than seven miles down, and it barely scratched the surface of our planet.
