Uncovering Earth’s Secrets: A Journey Into Its Deepest Hole

The Americans started this race to the bottom towards the end of the 1950s with the launch of Project Mohole. This was a plan to drill through the seafloor off the coast of Mexico’s Guadalupe Island, in order to reach the mantle of the Earth to take a sample.

One problem the experts faced when planning for this underwater drilling effort was the prospect of the vessel housing the drill spinning out of control. That could have been disastrous, but an innovative concept called “dynamic positioning” was employed to deal with the issue.

By 1961 Project Mohole had succeeded in boring a hole of 601 feet in depth at a depth of 11,700 feet below sea level. In another reality, that would have marked only the start of the project’s achievements, but, as things really played out, it was — ironically — its high point.

The Soviet attempt came after all this, in 1970. They started drilling in an area of Russia called Murmansk, which isn’t far from the border with Norway. Their project was the Kola Superdeep Borehole, though it actually consisted of several holes rather than just one.

You might wonder why any country would go to the trouble of initiating difficult projects like this. Why is it so important to dig deeply into the Earth’s crust? Was the whole thing just for bragging rights within the context of the Cold War? Or was it something more important?

Standing above it from the surface, the boreholes at the Kola site would have been quite unassuming. Their diameters were only 9 inches, which gave no sign of the holes’ true depth. But on and on they went, deep into the Earth.

In fact, you could stack Japan’s Mount Fuji — which stands at 12,388 feet tall — on top of Mount Everest, and you still wouldn’t have a measurement equivalent to SG-3’s depth. The borehole was deeper than the Mariana Trench, too, which, at 36,201 feet, is the open ocean’s lowest point.

As impressive as the Kola Superdeep Borehole was, though, we need to retain our sense of the Earth’s scale, too. SG-3 was deep, but on a planetary scale it was but a scratch.

And that’s just the crust. The Earth’s mantle, the layer which sits beneath the crust, is about 1,800 miles thick. And underneath that is the outer core, which goes on for roughly 1,400 miles. The inner core, which sits at the center of it all, has a radius of 758 miles or so.

Achieving this depth of 40,230 feet was an extremely time-consuming endeavor for everyone involved in the project. Drilling works kicked off on May 24, 1970, but it was almost two decades later that the record-breaking depth was finally reached.

The feat was achieved using a special technique known as rotary drilling, which the online magazine Interesting Engineering described in a piece published on January 2, 2024. The article’s writer, Christopher McFadden, explained, “[The project] used a branching structure, meaning the main borehole had several side holes branching off at various depths. This technique is widely used in the oil and gas industry.”

McFadden went on to explain that the drill bits employed for the task can be made from a variety of hardy materials, including diamond and tungsten carbide. But, tough as they were, their limits were tested by the extreme conditions they encountered. In order to deal with that, the engineers needed to think up some fixes.

The engineers’ innovations were very effective, but by 1989, at 40,230 feet, the project could continue no further. At this depth into the Earth’s crust, temperatures had become so hot that the equipment could function no more.

When you consider that, around this same point in history, humankind was successfully launching satellites into space, it can seem kind of strange to think we weren’t able to go much further beyond 40,000 feet deep into our own planet’s crust. After all, the Voyager 1 space probe that was sent to space in 1977 is now literally billions of miles away from Earth.

Things got especially difficult for the Kola Superdeep Borehole when the drill got to about 4.3 miles below the surface during the ’70s. Until this point, the equipment was able to cut through the granite in its path with relative ease. But here, at this depth, things got much harder.

The project continued as far as it could go, deeper into the Earth and its increasingly hot temperatures. The scientists were surprised by the intense conditions present past a depth of 10,000 feet, as the models they’d been working with up until that point had never suggested it would be like this down there.

The specific properties of the rock the scientists were boring through past a certain point were similarly surprising. After about 14,800 feet, the rock was more porous and permeable than expected. These surprising conditions of the rock, coupled with the hotter-than-expected temperatures down there, meant drilling became increasingly difficult.

The high temperatures were too much for the equipment to bear, so the depth of 40,230 feet was as far as they got. As noted, this depth was achieved in 1989. But that didn’t quite mark the end of the project, which pushed on for another three years.

If you ventured to the site of the project today, you wouldn’t find much of interest there. The place is little more than a ruin these days, the significance of the work that once took place there totally lost to time. The borehole itself is covered by a cap: you’d barely even notice it.

So, why is the Kola Superdeep Borehole considered to be so important? Why should people know about it? And, more broadly, what is to be gained from boring holes into the Earth’s crust at all? Well, there are many reasons.

Drilling incredibly deep holes into the Earth’s crust can help scientists to better understand geohazards such as volcanic eruptions and earthquakes. What’s more, the findings might shed new light on life on Earth and how it evolved.

Dr. Harms has spoken in more detail about the advantages seismologists specifically can gain from deep boreholes. He told How Things Work, “One example in detail is that observations very close to an earthquake zone allow [researchers] to monitor the initiation and propagation of even the tiniest earthquake in response to stress and strain.”

As for the Kola Superdeep Borehole specifically, there were numerous discoveries made. As we’ve already mentioned, the temperatures beneath the surface were way hotter than anyone ever expected, which meant the temperature map of the interior of our planet was updated.

As McFadden wrote, “One of the most surprising discoveries was the absence of the expected transition from granite to basalt between 3 and 6 kilometers below the surface. This transition, known as the ‘Conrad discontinuity,’ was previously believed to exist based on seismic-reflection surveys.”

Another major finding that resulted from the Kola drilling was that water, in its liquid form, is found at a much greater depth than it was ever previously thought possible. Dr. Harms told How Stuff Works, “One of the unexpected results was certainly the occurrence of open saline water-filled cracks documenting that the crust is not dense, but that pathways exist allowing fluids to flow.”

Arguably the most intriguing discovery made by the Kola project scientists was the suggestion of life at a depth of 3 or 4 miles beneath the surface. A series of fossils were found, some of which were around 2 billion years old. They belonged to unicellular marine organisms.

As well as signs of life, significant supplies of hydrogen were also noted by the scientists. The mud that emerged from the hole was, according to McFadden, once noted to be “boiling” with hydrogen. Apparently, the likely explanation for the presence of this hydrogen was something called serpentinization, which McFadden tried to explain.

That might not mean a lot to us laypeople, but experts were obviously fascinated to observe the hydrogen at the site of the borehole. The scientific community has so much to learn from the discoveries at the Kola Superdeep Borehole, even today. But not everyone has taken such a rational view of the work that was once going on there.

Some reports went on to try and back up the fantastical claim by asserting that audio recording equipment had detected a strange sound. It was almost like, the writers claimed, the awful din of human screams. 

Obviously a more rationally minded person would reject the claims that scientists had broken past the boundary separating our realm and Hell. But, if there really was a sound detected by the recording equipment, what could explain it? There must be an explanation.

Back to the actual science, and there’s the question of whether or not it will ever be possible to surpass the record depth set by the Kola Superdeep Borehole. According to Dr. Harms, it’s definitely possible. Just don’t hold your breath: it’ll take some time.

Ideally, scientists will eventually manage to bore down into the Earth’s mantle. That’s where some amazing discoveries stand to be made. We still have a lot to learn about the mantle, and much debate about it still exists today. Managing to reach it with a drill would be a massive step.

So, have any other projects of this nature been launched since the end of the Kola Superdeep Borehole? Well, attempts have been made to bore deeply into the Earth by Sweden, Austria, and Germany, but none have ever managed to reach the depth achieved by the Soviets.

The Chinese borehole, Shenditake 1, sits in the Taklimakan Desert, which itself lies in the Tarim Basin. At its inception, the project was intended to reach a depth of about 36,400 feet. That’s still less than the Kola Superdeep Borehole, whose record looks set to ensure for a while longer yet. Still, in terms of Chinese history, this effort is record-breaking.

The Tarim Basin, situated as it is in the middle of the Tianshan and the Kunlun mountains, is an extremely troublesome area for scientists to conduct exploratory works. Conditions there are very harsh, and the situation underground is complex.

Speaking to Xinhua, another expert from the Chinese Academy of Engineering expressed a metaphor to detail the issues facing the project. As Sun Jinsheng put it, “The construction difficulty of the drilling project can be compared to a big truck driving on two thin steel cables.”

According to The Guardian, the Shenditake 1 initiative is part of a wider effort on the part of China to break new ground in a range of areas. The country is hoping to explore space to a greater extent in future, for instance, while it’s also looking to lead the way in investigating the Earth itself.

There are parts of China which are already known to be rich in natural resources. The region of Xinjiang, for example, is home to oil and mineral deposits. And last year, oil and gas was discovered in the Tarim Basin itself.

But, of course, there’s much more to be gained from such deep drilling than simply discovering supplies of fuel and minerals. There’s much to be learned scientifically too, as the CNPC has itself acknowledged in a public statement.