The Kola Superdeep Borehole is designated as a Soviet Union science project, with no other objective than to see how deep a hole they could dig.
Drilling began on May 24, 1970 using a Uralmash-4E drilling rig.
The deepest borehole, SG-3, reached 40,230 feet below the surface in 1989 and is the deepest artificial point on Earth.
It was the longest borehole in existence until 2008 when it was surpassed by the 40,318 foot deep Al Shaheen oil well in Qatar, then again by the 40,502 foot deep Sakhalin-I Odoptu Op-11 Well off the coast of a Russian island.
The borehole got through the Baltic continental crust, which is about 22 miles deep. It reached rocks of Archaean age, more than 2.5 billion years old. Drilling stopped in 1992 where they reached temperatures of 356 degrees.