The Kola Superdeep Borehole is the deepest man-made hole on Earth, reaching a depth of 12,262 meters (40,230 feet) in the Kola Peninsula, Russia. It was drilled for scientific research to study the deep continental crust. The borehole provides valuable information on the composition, temperature, and pressure conditions of the Earth’s crust at great depths. 

Reference: Peive, A. A., & Krylov, D. P. (1988). “Results of Deep Continental Drilling at the Kola Peninsula, U.S.S.R.” Tectonophysics, 148(3-4), 241-246.