7 Kola Borehole facts

To date, the Kola Superdeep Borehole is the deepest point humans have ever made into the Earth. But how did they do it, and why?

By Christopher McFadden – Jan 2024

• Around the same time the world’s major powers were vying to make history in space, a very different scientific battle was also unfolding.
• Rather than stretching upwards, this battle competed to get as deep into the Earth as possible.
• The Soviet Union eventually won this battle with the Kola Superdeep Borehole, which managed to get as close to the Earth’s mantle as anyone before or since.

The strive for scientific excellence has been an important driver of human development. Throughout human history, it has led to some of the most important advancements in knowledge that, arguably, might not have ever been manifested without some good old-fashioned competition. One such field was the race to get as close to the Earth’s mantle as possible.

This race between major world powers ultimately led to the best effort our species has managed to date, the impressive Kola Superdeep Borehole. But what is it, and why is it so significant? Let’s find out.

1. There is more than just one borehole there

The Kola Borehole is a series of boreholes, not just one. Several boreholes were drilled there, the deepest reaching its record depth in 1989. This borehole is called SG-3 and is the deepest in the world. The Kola Superdeep Borehole reached over 7.5 miles (~12 kilometers), penetrating rocks over 2.5 billion years old.

This age dates back to the transition between the Archean and the Proterozoic periods of the Earth’s geological record. At this time, the Earth was very different from today, with the early surface largely covered with water. The earliest known life forms existing during the Archean were microbial mats called stromatolites that thrived in shallow waters.

The life forms that existed were mostly prokaryotes, comprising archaea and eubacteria. Photosynthetic processes, especially those of early cyanobacteria, emerged in the mid to late Archean era. These photosynthetic processes led to a permanent change in the ocean and atmosphere during and after the Archean era.

2. Scientists made some surprising discoveries

One of the most surprising findings during the Kola Superdeep Borehole project was the discovery of evidence of early life. At around 3.7 miles (~6 kilometers) depth, microscopic plankton fossils were recovered from the borehole samples. These rocks would be about 2.5 billion years old, making them some of the oldest fossils ever discovered on Earth.

Another interesting discovery was the presence of large amounts of hydrogen gas at depth. The drilling mud that flowed from the borehole was described as “boiling” with hydrogen. It has since been discovered that most of this gas was probably created through a process called serpentinization.

This is the process of hot water in the Earth’s crust (hydrothermal) altering minerals in rocks, transforming Fe-Mg (iron-magnesium) silicates such as olivine, pyroxene, or amphiboles contained in ultramafic (high-silica content) rocks into serpentine minerals. One of the byproducts of this process is the production of free hydrogen.

3. It is the deepest thing humans have ever dug

Drilling began on the Kola Superdeep Borehole on May 24, 1970, using the Uralmash-4E drilling rig and later the Uralmash-15000 series rig.

In 1979, the Kola borehole became the deepest human-made hole in history when it surpassed the previous record held by the Bertha Rogers hole in Washita County, Oklahoma, U.S., at a recorded depth of 31,440 feet (9,583 meters).

The Kola borehole reached 40,230 feet (12,262 meters) in 1989, making it the deepest human-made hole on Earth. As of 2023, it remains the deepest known hole created by human beings. To put that into perspective, the borehole is deeper than the Mariana Trench and deeper than Mount Everest is tall.

In terms of true vertical depth, it remains the deepest borehole in the world. It also held the record for being the longest borehole in the world for 20 years, but in 2008, it was surpassed by the Al Shaheen Oil Well in Qatar, which measures 40,318 feet (12,289 meters) along the well bore.

4. It was abandoned in 1992

The Kola Borehole was abandoned in 1992 after drilling encountered temperatures of 356 degrees Fahrenheit, °F (180 degrees Celsius, °C), exceeding the expected 212 degrees Fahrenheit (100 degrees Celsius). This surprisingly high temperature is caused by the heat flow from the much hotter mantle below the Earth’s crust. On average, temperatures rise about 72–87 °F for each mile (25–30 °C/kilometers) of depth.

At these depths, the rock density means that it acts more like a thick plastic than rock, making drilling almost impossible. It has been compared to keeping a pit in the center of a pot of hot soup. These high temperatures can also cause damage to equipment. As the Smithsonian explains, the hotter it gets and the more moisture in the environment, the more difficult it is to maintain the bore.

5. The borehole changed our ideas of the crust’s composition

The study of the Earth often relies on surface observations and seismic studies. However, the Kola borehole enabled geologists to directly examine the structure of the crust, putting their theories to the test.

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

Although a discontinuity has been detected beneath all of the continents, the drill at Kola never encountered the proposed layer of basalt. Instead, it found that the granitic rock extended beyond the twelve-kilometer point.

This discovery led scientists to realize that the seismic-reflection results were due to a metamorphic change in the rock, caused by intense heat and pressure, rather than a change in rock type as they had previously thought.

5. The project has been likened to geology’s “space race”

For around 50 years or so now, scientists have been engaged in a project to reach as far below the Earth’s crust as possible—ideally, to the Earth’s mantle. The scale and complexity of this task are so great that it has been likened to the global “Space Race” back in the 1950s and 1960s.

The mantle comprises 40 percent or more of the planet, which has a radius of 4,000 miles (~6,400 kilometers). It is located just below the Earth’s crust and above the Earth’s core and is thought to have a thickness of 1,800 miles (~2,800 kilometers). On the other hand, the crust is a relatively small part of the planet, averaging between three to 25 miles (~40 kilometers) in thickness.

The mantle is the major engine driving the planet’s constant evolution and contains a geological record of much of the Earth’s history.

If we could physically get some direct measurements and samples of the mantle, this would dramatically increase our understanding of how it works and, by proxy, give a better understanding of the Earth’s system as a whole. To this end, scientists in the United States attempted to reach the mantle in 1958 with “Project Mohole” by drilling through the Pacific Ocean floor off Guadalupe, Mexico. However, funding was discontinued in 1966 before the drillers reached the mantle.

In 1970, Soviet geologists took on the challenge and set their drills over the Kola Peninsula, which protrudes eastward from the Scandinavian landmass. This, as we now know, would become the most successful of all attempts, reaching the deepest so far.

6. It required some very specialist equipment

The project employed a drilling technique called rotary drilling, and it 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.

It employs a rotary drill bit attached to a drill string and a series of drill pipes that transmit rotational force to the bit from the surface. The drill bit is usually lubricated using special fluids called muds. These are circulated through the drill pipe and out of the drill bit and provide cooling, pressure control, cutting removal, and lubrication to aid the drill process.

Various drill bits are often made from tough materials like tungsten carbide or diamond. As the borehole deepened, challenges arose with torque, drill string weight, and temperature. These issues required innovative engineering solutions.

The Kola project had the unique feature of a branching structure, making the borehole resemble a sort of tree branch or root-like structure underground. At specific depths, side holes branched off from the main borehole. This allowed for a more thorough exploration of the Earth’s crust without starting a new borehole from the surface.

7. The borehole was finally sealed in 2008

Years after being mothballed, the borehole was finally sealed in 2008. Today, among the ruins of the support facilities on site, the only evidence of the site’s scientific significance is a small, now rusted metal cap. The cap is so nondescript you could be forgiven for not even noticing it if you ever visited the site for yourself.

This is a very real shame, as the technical and scientific achievement that the Kola borehole represents cannot be underestimated. It is probably one of the Soviet Union’s greatest contributions to scientific study ever. That being said, it is still revered as one of the Soviet Union’s most previous scientific relics, and rightfully so.

And that’s your lot for today.

El Pozo Superprofundo de Kola sigue siendo, hoy en día, el más profundo que nuestra especie ha explorado jamás en las entrañas de la Tierra. Si bien otros proyectos en desarrollo intentarán superarlo, por ahora, sigue siendo una de las perforaciones exploratorias científicas más impresionantes jamás emprendidas.