The deepest well ever drilled
Danilo Anton
In the Kola Peninsula, Northern Russia, the deepest well has been drilled. The initial goal of the Soviets (at that time it was the Soviet Union) when drilling began in 1970 was to reach the 15,000 meters depth taking samples and making measurements allowing to understand a little more the nature of the earth's crust.

The Soviets chose to drill the superdeep borehole on the Kola Peninsula in the far north.
Drilling began in 1970. In the first four years it progressed rapidly and the well reached 7,263 meters.
Up to 7,000 meters, the Soviets could use standard equipment from the oil and gas industry. From that point down, because of the absence of any previous drilling reference at that depth, they had to develop new techniques and machinery, using the method of trial and error. The Soviets had to face many difficulties, but the main problem encountered were the high temperatures to which the drill had to word. The Soviets had to devise refrigeration systems and drills capable of working at over 300 degrees Centigrades.
After almost 13 years of work, in 1983, drilling reached 12,000 meters. The work stopped because of a problem a year later.
In this situation, it was decided that before proceeding with the drilling it was necessary to make the hole wider and place a "wrapper", an outer pipe, to stabilize the cavernous sections of the top of the well, which were the most volatile. The wellbore was extended up to 295mm in diameter and an outer pipe placed 245mm to 8,000 or 9,000 meters deep. During the operation to make wider the well, at the depth of 7,000 meters, the hole deviated from its previous path and continued drilling a new lateral hole 295mm in diameter. Once it was finished the placement of the outer pipe, drilling continued with a diameter of 215.9mm.

It would take almost 5 years to reach the previous depth at break, but in 1989 they reached 12,262 meters. During that year, it was expected that the well could reach 13,500 and in another four years, 15,000. However, the temperatures were found very different from those expected. Unlike what happened during the first 3 kilometers of drilling, where the temperature coincided with the predictions (just one degree of increase per 100 meters), from that depth, the increase of temperature began to be much faster
A 10,000 meters temperature had already reached 180 degrees, far more than the 100 degrees that had been predicted. If the progression of the temperature followed at 15,000 meters it would be about 300 degrees, which would have prevented the drill work. With these data it was considered impossible to continue drilling and in 1992 it was decided to stop work
Nevertheless, Kola drilling could be considered a success. It had gone through a third of the Baltic continental crust, which is supposed to be about 35 kilometers thick, and allowed exposing rocks 2,700 million years old. Scientists had conducted numerous geophysical studies of the deep structure of the Baltic plate, seismic discontinuities and the thermal regime of the Earth's crust.
But one of the most fascinating discoveries was the fact of not finding change of seismic velocities in the hypothetical transition from granite to basalt within the crust. This theory was proposed by geophysicist Harold Jeffreys, and that to that  date was accepted as a working hypotheses by most geologists. According to the theory of the British, the change of seismic propagation velocity within the crust was due to the passage of granite to much denser basalt.

Surprisingly, at that depth there was a passage from granites to metamorphic granites, and no basalts were found. It was precisely at the bottom of this layer where the expected change had to take place, but this change did not happen. Additionally, the rocks of the lower layer were very fractured and saturated with water. Finding water at these depths was also entirely unexpected. Geologists believed that water, unlike surface water, should come from the deeper layers of the cortex, but that it had been impossible to reach the surface because of a layer of impermeable rock.
Another unexpected discovery was the large amount of hydrogen, contained in a muddy sludge. The mud seemed to boil with hydrogen escaping from it-  It was also surprising to find that the density of the rocks decreased in greater depth. Near the surface, the density tends to increase with depth, but at a depth of about 4500 meters a sudden drop in the density and an increase in their porosity and permeability was recorded.