Some interesting discoveries and anomalies resulted in the deep excavation. One was boiling hydrogen - actually the mud underneath boils due to evaporation of hydrogen. Temperature was discovered to be about 190 degrees celcius. Such high temeperature prohibited further drilling, so we may think it might go deeper.
The first such project started in the USA in around 1960, but it stopped due to lack of funding. That sounds strange to me as the USA does not lack funding on anything. So the fact might be that, pre-researches or feasibility studies might show worthless discoveries and instead investment of funding to some other discoveries might have been more worthy of.
Alongside the drilling, another putative hocus pocus was going on that some other scientists in Russia drilled about 15 Kilometers below the earth where temperature reaches about 1100 degrees celcius. They said the deeped down a heat resistant microphone and recorded strange sounds underneath that sounded like something supernatural - as if screams of punished people as in hellfire as described in some religions. It may be noted that, the location of hell is described as to be in the core of earth in some religions. I think there is a movie "Nine Miles Down" that explores this phenomenon where deeping down so deep reveals a demon underneath - or something like this. This hoax is called putative as it was even supported by broad media coverage, and it is named 'Well to Hell'.
It may be noted that in their song "Cluster One", British band Pink Floyd used some sound effects that is said to be sonographically recorded sound of the clinking materials floating at the core of the earth. You might know, the core of the earth has zero gravity and it is extremely hot there and only some heat-resistant materials float there in a melted environment. But this should not be confused with the 'Well to Hell' hoax sound.
Anyway, this clearly shows the power of engineering. Digging deep enough for 12 kilometers ain't a cakewalk at all. Try to visualize a road up to 12 kilometers, and then move that road vertically into the ground - then only you will understand what it means drilling 12 kilometers into the ground. Obviously that's a nice piece of engineering.