At the Kola Institute, pictured, the Russians drilled for more than 15 years to reach a crust depth of 40,226 feet, a record that's never been broken. But however successful the mission was as an exploration, the geological findings from the site remain murky and obscured by the way they emanated out of the fading Soviet scientific machine.
Stanford geologist and drilling expert, Mark Zoback, said that the Kola borehole was "an anomaly" even within the rather grandiose field of superdeep drilling projects.
Read how they did it at Wired. http://www.wired.com/science/discoveries/multimedia/2008/08/gallery_kola_borehole