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.