Conditions at the bottom of a VERY deep hole I'm a science fiction author looking to describe what conditions might be like at the bottom of a cylindrical shaft ~ 30 meters across and ~ 35 km deep.  It's located somewhere on the edge of Yellowstone national park (if that matters). The top is open to ambient, albeit controlled, conditions in a great big lab room. If you drop a coin down this well, there's nothing man-made in the way of it getting to the bottom.

The shaft itself uses advanced unobtanium wall liners, so the engineering challenge of keeping it from collapsing or otherwise self-destructing has been taken care of. I'm not sure how insulating the unobtanium is from the surrounding rock right now, could be "none" to "completely," depending on how interesting that makes what's happening at the bottom.

I'm pretty sure the atmosphere at the bottom will be amazingly hot and dense, probably quite a bit denser than several hundred feet of water, but I'm hoping someone more knowledgeable can help out with the details.

Contributors will definitely get a mention in the acknowledgements. Big contributions will likely result in a signed copy of the final book. This will be the third book in the Gemini Gambit series, which will likely hit the shelves in late 2017/early 2018.

UPDATE The hole is used as part of a super-massive geothermal power plant, so hot is good. Dense is good. I'm thinking the convection potential would perhaps make mounting ducted fan windmills somewhere also good for generating power.
If it is enclosed and undisturbed there would be a risk of the heavier gasses collecting at the base of the hole, with carbon dioxide, argon, neon and xenon displacing the nitrogen and oxygen. Hence the danger of asphyxiation for any organism at that depth. Mixing, to keep the air homogeneous would also have the effect of heat transfer to further up the hole, so you will need artificial mixing and cooling. There are likely to be traces of helium and hydrogen at such depth, but not enough for the upward flow of such light gasses to have any significant mixing effect. However, hydrogen will be a potentially explosive problem at the enclosed surface environment. At these temperatures water rock reaction at the exposed base of the hole would be rapid. It is not clear whether your hole would have punched through to the mantle. I'm assuming it would, in which case there will be water vapour - hartzburgite reaction to yield more hydrogen. Serpentinization of the rock will proceed, involving about a 17% rock expansion. The rate of such a reaction will be limited by the amount of water vapour in the hole. A by-product of the reaction will be moisture at pH12 which will rapidly react with the high partial pressure of CO2 to produce carbonate encrustation. So it will be a problem to keep the hole clean - unless you prevent any moisture entering the hole from the surface control area.