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Kola Superdeep Borehole

The Kola Superdeep Borehole is the world’s deepest borehole. Since the beginning of drilling in 1970, this borehole has been a site of extensive geophysical studies. It was cored nearly continuously from the surface to its total depth of 12.26 km.

To scientists, one of the more fascinating findings to emerge from this well is that the change in seismic velocities was not found at a boundary marking Jeffreys' hypothetical transition from granite to basalt; it was at the bottom of a layer of metamorphic rock that extended from about 5 to 10 kilometers beneath the surface. The rock there had been thoroughly fractured and was saturated with water, which was surprising. This water, which unlike surface water must have come from deep-crust minerals, had been unable to reach the surface because of a layer of impermeable rock.

Another unexpected discovery was the large quantity of hydrogen gas, with the mud flowing out of the hole described as "boiling" with hydrogen.