|MadSci Network: Earth Sciences|
First of all let me qualify my answer. The details may be a little wooly, but the answers will point you in the right direction to find the full information.
As with any "human endeavour" type of question, the Guinness Book of Records is probably the best place to look for the "right" answer. Let me give you my Two cents worth, though...
The deepest research borehole ever drilled was in Russia, on the Kola peninsula. Over a period of more than a decade a huge purpose-built rig drilled to over 12 kilometres to investigate the structure of the Continental Crust (sorry about the units, I'm in Australia and we're metric here - so are the Russians).
The results showed unexpected fractures and fissures and water-bearing zones deep in the crust, where conventional wisdom expected the pressure of rock above to close up all cracks.
The deepest oil wells that I know are drilled to about 6km, say 20,000ft. Most normal oil wells are drilled to about 3000 to 5000 metres -10,000 to 16,000 ft.
I don't know about water wells, but 1000 years ago in Sichuan, China, the locals had developed bamboo and rope operations with steel tools that enabled them to drill to 1000 metres in search of saltwater and natural gas. They burned the gas to evaporate the water and concentrate the salt for sale, far inland away from the sea.
For mining, the South African Gold mines are probably the deepest. They go so deep that the natural rise in temperature with depth in the Earth becomes a serious problem. Despite cooling water, the mines are unpleasantly hot.
There was a project to drill through the Earth's crust - the Moho project. This developed into the Deep-Sea Drilling Project and the modern Ocean drilling Project. Although relatively shallow holes are drilled (seldom more than 1000m), the knowledge of oceanic sediments and volcanism has transformed out understanding of the world. In any case, we have since learned that in many areas slices of deep crustal and mantle rock are thrust up in Ophiolite belts and are brought to the surface for us to see without drilling.
The deepest holes in the crust are natural. Volcanoes bring up material from tens of km depth, and kimberlites, the exotic rocks that contain diamonds, appear to have arisen from the Mantle in a matter of a few hours (Otherwise the Diamonds would have broken down)
In subduction zones the crust is depressed, and the deepest ocean depth is in the Marianas Trench, over 11km depth, compared to about 6km in the adjacent unsubducted oceanic plate.
At Vredefort, in South Africa, a huge circular structure - the Vredefort Dome, is interpreted as a huge astrobleme (impact/crater structure) hundreds of km across and bringing roks from 25km deep up to the modern surface. It is speculated that a huge meteorite - perhaps of asteroid size - hit this area over 2,000 million years ago, and blasted an enormous hole in the crust. The hole was so deep and wide that the crust flowed back in to fill it up, bringing the deep crustal rocks to surface.
Do some reading on these topics and see what else you can find.
Nick Hoffman Geophysicist Extraordinaire "Everything has a Disclaimer on it these days"
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