Any time you are digging a narrow shaft into the ground, the hole in the Earth you are creating is called a borehole. There are many reasons to create a borehole such as finding oil, water, or precious metals. Boreholes can also be dug to act as temperature proxies giving scientists data on temperatures from the past.
The Kola Superdeep Borehole
The deepest borehole on Earth is the Kola Superdeep Borehole. Drilling began on May 24th, 1970, and many different boreholes were drilled, branching from a central shaft. The deepest of the boreholes is SG-3, which was completed in 1989, and is 7.62 miles deep (12.26km). SG-3 is the deepest hole ever drilled. The longest hole ever drilled, however, is for an oil well in Qatar which edges SG-3 out ever so slightly at 7.64 miles.
Work was planned to continue on to 15 km (9.3 miles), but temperatures at lower depths were much higher than expected. At the bottom of SG-3, temperatures of 100° Celsius (212° Farenheit - the boiling temperature of water) were expected, but instead they found the temperature to be nearly twice as high, 180° C (356° F). Drilling deeper was found to be nearly impossible, because of the havoc the high temperature would wreak on the drill, and work on the project stopped in 1992.