The Kola Superdeep Borehole

The deepest point beneath the Earth's surface created by humans is an experimental borehole drilled by the Soviet Union between 1970 and 1989 in the Kola Peninsula in northwest Russia. You can read more about it here:
Kola Superdeep Borehole

The deepest spur of the borehole, labelled "SG-3" by the project's geophysicists, bottomed out after 19 years of on-and-off drilling at a depth of 12,262 meters (40,230 feet for those who don't like moving decimal places to convert units). This is short of the original depth goal of 15 kilometers, first due to mechanical problems with the drill and then from unexpectedly high temperatures in the well. In 1984 a 5-kilometer-long piece of drill string failed and was abandoned in the central hole. The project was forced to backtrack the length of the broken drill string and start drilling again off a side borehole, but when the temperature in SG-3 reached 180 degrees Celsius further progress was deemed unfeasible and the project fell into disarray amid the collapse of the Soviet Union and the painful depression that followed.

Propaganda value was part of the reason why the Soviets chose to dig so deep at Kola, since there was no equivalent western program that could claim to reach deeper into the Earth. That said, a good deal of geophysical knowledge came from the Kola borehole during its prime, including a better understanding of the transitions that happen in rock structure deep within the Earth's crust and the discovery of large amounts of water and hydrogen sequestered far below the surface. It's a shame an earlier American program to reach deep under the oceanic crust faltered due to a lack of funds.

Nowadays the deepest wells beneath the surface are drilled for profit rather than science. Before its horrifying and public demise the Deepwater Horizon platform drilled to within a mile of the depth of the Kola borehole to extract oil lurking beneath the Gulf of Mexico. Drilling technology has advanced so quickly that getting the oil out of the ground wasn't anywhere near as difficult as stopping it once it the flow was started. The era of herculean effort to sip petroleum won't last forever, though, and hopefully this technology will be applied to exploration in the open-ended sense once again. After all, something seems odd about a people who have touched the Moon but never reached very far under their own feet.