In terms of depth below the surface, the Kola Superdeep Borehole SG-3 retains the world record at 12,262 meters (40,230 ft) in 1989 and still is the deepest artificial point on Earth.
Don't mind the 9-inch diameter. Instead, focus on the Kola Superdeep Borehole's unmatched 7.5-mile depth. Started in 1970 by Russian scientists on the Kola Peninsula of Russia ultimately became the deepest hole in the world—deeper than even the deepest part of the ocean—after about 20 years of digging and experimentation. The 356 Fahrenheit temperature at that depth, however, made it impossible for tools to keep going. The site has been abandoned since 2008, and the hole bolted shut so nothing can get in.