The deepest a human has been underground is approximately 12 kilometers. This depth was reached in 1989 by Soviet miners working on the Kola Superdeep Borehole project in Russia. The borehole is still the deepest artificial point on Earth.
The Kola Superdeep Borehole was started in 1970 and took almost twenty years to reach its final depth. The miners used a special drill that could bore through rock at a rate of over five meters per hour. They had to constantly pump out water and other fluids to keep the hole from collapsing.
At depths of around seven kilometers, the temperature of the rocks starts to increase due to the pressure and heat from the Earth's mantle. By twelve kilometers, the temperature was estimated to be around 180 degrees Celsius (356 degrees Fahrenheit). The miners had to use special cooling systems to keep their drill bits from melting.
Although they only reached a depth of twelve kilometers, the Kola Superdeep Borehole provided scientists with valuable information about our planet's geology. It also showed that it is possible for humans to survive and work at extreme depths underground.