What was learned in the deepest borehole never drilled
On the Kola Peninsula, in northern Russia, is the deepest well ever drilled.

The initial objective of the Soviets (at that time was the Soviet Union) when they started drilling in 1970 was to reach up to 15,000 meters deep taking samples and making measurements that would allow a little more understanding of the nature of the earth's crust. They also sought to cross the discontinuity that separated the upper crust from the lower one.
They did not succeed because temperatures rose faster than expected. Already from 7,000 or 8,000 meters there were 160 degrees, and more than 200 degrees at 12,000 meters. They found no discontinuities, no basaltic rocks or igneous rocks as expected. It was a succession of metamorphic rocks (granite type). It surprised that at those depths there was water and that large volumes of hydrogen emanated (maybe methane, too). These results confirm the hypothesis of the recently deceased Spanish geologist, Vicente Sánchez Cela, that rocks of granite composition underlie the continental shields and Thomas Gold's hypothesis, that hydrocarbons and hydrogen abound in depth and that none of these compounds / elements they are of biotic origin (fossil fuels) and that both methane and free hydrogen come from the earth's mantle

Drilling
The Soviets chose for the drilling of their super-deep well a place on the Kola Peninsula in the far north.
The drilling began in 1970. In the first four years it advanced rapidly and the well reached 7,263 meters.
Down to 7,000 meters, the Soviets were able to use standard equipment from the oil and gas industry. From that point on, as there was no previous reference to drilling at these depths, new techniques and machinery had to be developed, using the trial and error method. The Soviets had to face many difficulties, although the main problem they encountered was the high temperatures at which the drill had to work, which meant that refrigeration systems and drill bits capable of working at more than 300 degrees had to be devised.
When it had been almost 13 years of work, in 1983, the drilling reached 12,000 meters, but the work was stopped by a breakdown a year later.
Faced with this situation, it was decided that before continuing with the drilling it was necessary to make the widest hole and place an "envelope", an external pipe, to stabilize the cavernous sections of the upper part of the well, which were the most unstable. The diameter of the well was extended to 295mm in diameter and an external pipe of 245mm was placed up to 8,000 or 9,000 meters deep. During the operation to make the well wider, at a depth of 7,000 meters, the drill deviated from its previous trajectory and continued to drill a new lateral hole of 295mm in diameter. Once the exterior pipe was finished, the perforation continued with a diameter of 215.9mm. It would take almost 5 years to reach the depth before the break, but in 1989 it reached 12,262 meters. During that year, it was expected that the well could reach 13,500 and in another four years, the 15,000. However, the temperatures that were being found during drilling were very different from those expected. Unlike what happened during the first 3 kilometers of drilling, in which the temperature coincided with the predictions (just one degree of increase per 100 meters), from that depth, the increase in temperature began to be much more Quick.
At 10,000 the temperature reached 180 degrees, much more than the 100 that had been predicted. If this progression continues, the temperature at 15,000 meters depth would have been about 300, which would have prevented the drill from working. With these data it was considered impossible to continue with the drilling and in 1992 it was decided to stop the work. In spite of everything, the perforation of Kola could be considered a success. It had crossed a third of the Baltic continental crust, which is supposed to be about 35 kilometers thick, and had allowed to come to light rocks of 2,700 million years old. The scientists had carried out numerous geophysical studies on the deep structure of the Baltic plate, the seismic discontinuities and the thermal regime of the earth's crust.
But one of the most fascinating discoveries was the failure to find change of seismic velocities in the hypothetical transition between granite and basalt within the crust. This fact overturned the theory of the geophysicist Harold Jeffreys, and that to date was accepted as a working hypothesis by most geologists. According to the British theory,