The SG-3, Kola Superdeep Borehole, or Zapoliarny borehole, named after the nearest town, or by its official name the Kola Superdeep Borehole Experimental Reference Well (SG-3), is a borehole drilled from May 24, 1970, to 1989 in the USSR, on the Kola Peninsula.
This borehole is the deepest borehole in the world at 12,262m (actual vertical depth), but its opening was permanently sealed in 2005, without ever reaching the goal that the researchers had set for themselves: to drill down to 15 km deep. While they expected to encounter temperatures of around 100 °C, the rocks were much closer to 180 °C.
Not being prepared for this extreme heat, they had to stop drilling and seal the opening. The cover of the Kola Superdeep Borehole (welded in place) in August 2012.
In total length, it was, however, overtaken in 2008 by the Al Shaheen oil well measuring 12,289 meters long (40,318 feet) in Qatar, and in 2011, by the Sakhalin I Odoptu OP-11 well measuring 12,345 meters long (off the Russian island of Sakhalin).
