This is something I have wondered for some time now. What is the deepest hole ever dug by a human? I assume it would be a hole dug searching for oil, but I could be wrong.
Thank you in advance to all who reply 
Neat, but how wide is it? Could I dive headfirst down it? Or does it taper down to six inches wide after a couple of hundred feet? If I filled it with water, could I run a steam-powered generator off it and have free electricity for the rest of my life?
You have to define what you mean by “dug”. Boreholes are, by definition, “drilled”. When I think of digging, I think of a shovel, that is, something that breaks up the earth, yet stays on top of it, and uses the actual implement to lift the dirt out without rotation. If you exclude drilled holes, the next most likely answer would be an underground mine. Although, technically, many of those are “drilled”, also, albeit by a different type of drill. The third deepest hole, but the one that fits the definition for dig that I have given, would seem to be an open pit mine that uses a drag line to scoop out dozens of cubic yards of earth at a time.
Once, near Zapolyarny, for 20 minutes, in 1962.
I am having a hard time finding a picture of it but the Russian hole destroys our ideas about digging down very deep or digging to China for that matter. The Russians tried everything including digging a giant pit to help go deeper and deeper. The earth gets extremely hot very fast and present day drilling techniques won’t allow drilling at anything more than 0.1 percent of the earth’s diameter. The drilling tools simply fail at the heat and, more importantly, the earth becomes plastic and can’t be stopped from filling in the hole. It isn’t simple dirt all the way down and it is unlikely that we could ever overcome this to dig significantly deeper.
I didn’t see your first question get answered, but I was looking up information about the hole and the figure was just over 8 inches wide. That may have been a wiki figure, or it may have been one of the other websites I was looking at. It was late and I’ve slept since then!
19 years of drilling, and they only made it 1/3 of the way through the Earth’s crust. Guess we can forget about the direct route to Australia.
Great quote:
Those scientists really know how to party. 
Greensburg Kansas, the town that was nearly wiped off the map by a tornado earlier this year, claims to have the world’s largest hand-dug well.
I think “superdeep borehole” would be great insult to toss at some insufferable person at a party.
How about the Mohole?
Just under 8 1/2 inches.
The wells look a bit like a telescope with the fat end up at surface. A hole of say 36 inches is drilled to get past the surface sediments, then steel pipe of say 20 inches (known as casing) is run into that hole. The space between the original 36 inch hole and the outside of the 20 inch casing is filled with cement.
Then drilling commences with a smaller drill bit that will fit inside the 20 inch casing.
This hole is drilled down to a depth where the formation pressures or the stability of the rock becomes a problem. At this point, more steel pipe is run into the hole (smaller than 20 inches) and it is cemented in. Drilling commences with a smaller drill bit, and so the cycle continues with the hole getting deeper and smaller.
I do not have the exact casing sting design and depths, the above sizes are for illustrative purposes. However have a look here for details and some schematics that may explain a little better from a similar project in Germany.
See page 9 for a schematic - NOTE PDF
http://www.slb.com/media/services/resources/oilfieldreview/ors95/jan95/01950422.pdf
The main limitations to getting deeper as mentioned by Shagnasty is the wellbore stability and the temperature. Casing drilling and expandable casing may help, at least in preventing the stepping down of the hole size and being able to seal off plastic formations without loosing a hole size.
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