Drilling deep holes into the Earth’s crust allows scientists to study the composition and structure of the planet’s interior. The deepest hole drilled to date is the Kola Superdeep Borehole which reaches 12,262 metres (40,230 ft) below the surface. This incredible feat of engineering has provided geologists with valuable data about the continental crust and prehistoric life forms. But even more intriguing is what was found at the bottom of the world’s deepest hole.
The Kola Superdeep Borehole
The Kola Superdeep Borehole is located on the Kola Peninsula of Russia. Drilling began in 1970 and continued intermittently until the 1990s. The stated goal was to penetrate deep into the continental crust to learn more about its composition and properties. The project was largely funded by the Soviet government for scientific research purposes.
Several important discoveries were made during the drilling process. As the borehole passed through the crust and into the upper mantle, scientists were able to analyze rock samples and temperatures at depths never before reached. The project provided the first real insights into the structure of deep continental crust.
But despite reaching an unprecedented depth, the Kola drill hole remained far short of its original target depth of 15,000 metres. Challenging drilling conditions including high temperatures and fractured rock forced an end to the exploration at the 12,262 metres mark in the early 1990s.
Finding Fossil Water
In the late 1980s, as the borehole approached 10,000 metres deep, an unexpected discovery was made. The drill suddenly dropped into a cavity and a fountain of hot water sprayed up from below. This water was determined to be “fossil water” trapped inside the rock since the earliest days of the Earth’s formation.
The ancient water originated from a time when the Earth’s surface was still very hot and hostile. As the planet cooled, this primordial water became trapped within fractures and cavities in the rock. The Kola borehole had managed to intercept one of these isolated reservoirs. At up to 4 kilometers deep, it represents the deepest fossil water ever found.
Analyzing the chemical composition of the water proved that it matched no known sources from the surface. It also contained high amounts of dissolved hydrogen due to a reaction with the surrounding iron-rich rock. The discovery demonstrated that ancient isolated aquifers can exist even deeper in the crust.
Finding Microscopic Life
In addition to fossil water, the Kola borehole made another unexpected discovery in the form of microscopic plankton fossils. As early as six kilometers down, drilling fluid returned to the surface containing the remains of fossilized diatoms and other organisms. This proved that ancient microbial life had existed deep in the isolated water trapped within the rocks.
The fossilized plankton found are known to date back millions of years. This rare discovery added to evidence that microorganisms could live deep underground without access to sunlight. It also expanded the known habitat for ancient plankton. Scientists theorize that before the surface of the Earth cooled, these organisms proliferated in the warmer, mineral-rich water circulating through the interior.
Up until then, such fossils had never been found at significant depths. Their presence in the Kola samples represented a major discovery in the study of subsurface and ancient life.
Reaching High Temperatures
Aside from revealing fossils and isolated water, the Kola Superdeep Borehole provided temperature data from deep within the continental crust. Temperatures rose rapidly with depth due to the geothermal gradient. Measurements reached 356°F (180°C) at the bottom depth of 12,262 meters.
Such high underground temperatures cause heat flow into the borehole. This influx of heat added to the drilling challenges and required cooling to counteract it. The temperature data gave scientists clearer insights into the Earth’s internal heat engine. It also allowed mapping temperature profiles within the deep continental crust for the first time.
The temperature trends pointed to localized hot zones as opposed to uniform heating. This was indicated by an irregular upward spike in temperatures at certain depths. The reasons for these hot zones are still being investigated, but may relate to water or chemical interactions. Overall, the project yielded the most accurate temperature measurements ever recorded from the deep crust.
Finding Geological Anomalies
Several interesting geological anomalies were found by analyzing the drilling samples from different depths. At about 6,700 meters down, a sudden change occurred from granite to basalt layers. This did not match the expected structure and composition. The underground rock was also more fractured and porous than predicted by existing models.
One major surprise finding was tiny ultra-hard diamonds found within the basalts at around 10,000 meter depths. These diamonds were believed to have formed with the rock at extremely high pressures. Their presence was a mystery since such diamonds were not expected so deep into continental crust.
Other abnormalities like high hydrogen and helium gas concentrations have still not been fully explained. The unusual geology indicates our knowledge of deep continental crust makeup is still far from complete. The Kola borehole provided scientists with plenty of new puzzles to solve.
Table of Kola Borehole Depth Findings
| Depth Reached | Finding |
|---|---|
| 0 – 6000 m | Granite rock |
| 6700 m | Transition to basalt rock |
| 9000 m | Fossilized plankton |
| 10000 m | Isolated diamonds |
| 12000 m | Record temperature 180°C |
Conclusion
The Kola Superdeep Borehole remains the deepest hole ever drilled by humans. It penetrated about a third of the way through the continental crust, but drilling was stopped well short of the original 15,000 meter goal. Even so, these record depths provided scientists with groundbreaking data about the composition, structure, and history of the Earth’s interior.
Fossil water and microscopic fossils were just two of the surprising finds proving isolated biospheres can exist deep underground. Other discoveries like diamonds and geological anomalies forced researchers to reconsider their models of the crust. Although the Kola borehole never reached the Earth’s mantle, it demonstrated that a great deal of mystery still resides far beneath our feet.