Kola Superdeep Borehole

SG-3 was drilled on the Kola Peninsula in north-western Russia as a scientific effort to study the composition and properties of continental crust at depth. Drilling began in 1970. The borehole reached 12,262 metres (about 12.3 kilometres) in 1989, a record for vertical depth below the surface. It is a narrow borehole rather than a shaft, and its depth is small compared with the roughly 6,371-kilometre radius of Earth.

The drill recovered samples and enabled measurements of rocks and conditions deep in the crust. Studies of SG-3 core have contributed to research on ancient basement rocks and fluids at depth. For example, a later study examined quartz veins and fluid inclusions in core from roughly 9.1 to 10.7 kilometres, reporting gold nanoparticles in those inclusions. Such findings concern particular samples and do not mean that the borehole encountered a mineable gold deposit. The project also informed understanding of how temperature and rock behaviour at depth can differ from predictions.

Progress was constrained by the increasing temperature, difficult drilling conditions and the expense of sustaining a specialised scientific programme. Drilling operations ended in 1992, after temperatures of about 180 °C were measured at depth, substantially hotter than anticipated. The site was subsequently closed. Accounts sometimes portray the hole through sensational legends, but these are folklore rather than scientific findings.

SG-3 did not reach the mantle, nor was it intended as a route through Earth. Its scientific value came from direct sampling and measurements at depths rarely accessible in continental crust. Together with other deep-drilling projects, it illustrates both the opportunities and practical limits of investigating Earth’s interior by drilling.