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Assuming a fairly earth-like planet, perhaps, higher in minerals and resources. And a race of highly industrious, highly cooperative, very enginuitive people. Their science is comparable to our own, but they are total capable of using that science to engineer solutions to the challenges that will ensue.

They begin building their technological wonder. They begin mining out a very large mineshaft, directly down.

They reach the point where the planets crust becomes scolding hot. They then construct a pipeline to the surface so that they can pump in sea water. They cool the rock by hosing it down and then continue mining.

They then reach a point, where the shaft walls are heating up above them. So rather than posting people at each interval with hoses to prevent the wall from collapsing, they create some kind of automated device that mounts to the wall. It pipes in water, heats it against the rocks and then pipes out steam. The steam is sent up to the surface via a high pressure pipe. This steam pressure can be harvested by steam turbines to generate electricity. This good because powering the lift that is used to remove the rubble from the bottom of the shaft is taking allot of power.

So they continue building this shaft down, with the walls fully encased with the steam producing rock coolers. The sky is completely overcast because of all off the evaporated sea water. (They must also have to send up all of the salt at this point. Obviously, the steam and water pipes branch off at each level so that there are just the two main pipes.)

Eventually they get close enough to the planets core that the composition of the matter that they are mining is mostly heavy metals. They stop Physically mining further and use ceramic pipes to send molten metal to the surface under it's own pressure.

Mission accomplished, they now have access to a continuous tap of iron, lead, gold, silver, uranium, or whatever other heavy mineral they need, depending on the composition of their planets core.

physically possible?

conceivably feasible?

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    $\begingroup$ I think, at it's simplest, this is how a volcano forms... $\endgroup$
    – Frostfyre
    Commented Jul 31, 2015 at 3:22
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    $\begingroup$ @Frostfyre volcano is formed when continental plates collides due to convection of molten mantle, gas pressure builds up over times inside a volcano will create ernomous force to throw the top layer of rocks hundreds of meters high anyway since you mentioned the word simplest I'll be generous +0.55 (you're lucky system don't accept decimals) $\endgroup$
    – user6760
    Commented Jul 31, 2015 at 5:20
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    $\begingroup$ I don't understand why you gave him an up vote. A volcano is formed in a fundamentally different way to the process described in my question. Like with heating and force from bellow, rather than cooling and physical digging from above. $\endgroup$
    – Lorry Laurence mcLarry
    Commented Jul 31, 2015 at 5:48
  • $\begingroup$ I was thinking of lava tubes and magma moving up from the mantle, not the act of forming the hole. Still, my quirky comment wasn't entirely serious and was never meant to be scientifically accurate. $\endgroup$
    – Frostfyre
    Commented Jul 31, 2015 at 12:02
  • $\begingroup$ Putting aside for a moment the daunting scale of the temperature problem, and the even more pressing (heh) pressure problem that some answers have mentioned, at some depth the structure of your tunnel walls will collapse under its own weight. See also: Why you can't build a tower to orbit. $\endgroup$
    – Doug Warren
    Commented Jul 31, 2015 at 16:05

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I don't think this can work: the problem is not just temperature but pressure. The walls of the tunnel need to be made of a substance that is much more resistant to temperature and pressure than the natural mineral (molten or otherwise) substance of the mantle. So, building the tunnel out of cooled-down mantle substance is doomed to fail, i would say. You need some unobtainium. https://en.wikipedia.org/wiki/Unobtainium#Science_fiction

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    $\begingroup$ Btw, that shiny stuff that cutlery is made out of is called metal. "mettle" is a different word similar in meaning to "strength of character or bravery". $\endgroup$
    – Max Williams
    Commented Jul 31, 2015 at 15:45
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    $\begingroup$ I guess I didn't really think about the pressure... $\endgroup$
    – Lorry Laurence mcLarry
    Commented Aug 19, 2015 at 1:59
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Possibly possible, but not effective.

  • It will get uncomfortably hot long before the rock turns liquid.
  • Somewhat plastic but not yet molten rock will squeeze your bore shut. Can you armor against those forces when the armor is riddled with cooling pipes?
  • Iron is concentrated at the core, but that is thousands of miles down.

This sounds like a space opera project, not near-future science fiction.

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    $\begingroup$ well... if the hole is cylindric, and sufficiently cooled, it might just be able to withhold the pressure from the surrounding material that is above its glass temperature. Provided you can carry away enough heat with those coolers, that is. $\endgroup$
    – Burki
    Commented Jul 31, 2015 at 14:03
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I don't know if you'd actually need unobtainium. Wikipedia puts the pressure at the earth's core at 300 - 360 GPa here. The compressive strength of diamond is speculatively more than 100 GPa. Without trying to do the engineering on collapse pressure, a cylinder can very vaguely withstand pressure of the same order of magnitude as it's materials's compressive strength.

So, conceivably 3 or 4 concentric cylinders of diamond with high pressure gas between them could withstand this load. The inside is at your room pressure, which will working pressure, which will be several hundred atm (air pressure goes up by roughly 1 atm per 9km of descent into the earth). Each shell supports a pressure difference just below its failure strength. The spaces between the tubes could be your cooling fluid circulation, after all, high pressure steam shouldn't be in short supply.

Of more realistic materials, tungsten carbide has a compressive strength of 5GPa, so you'd need something like 60 or 80 concentric cylinders.

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    $\begingroup$ Can you account for how to handle the immense temperatures at the earth's core? $\endgroup$
    – Green
    Commented Aug 20, 2015 at 19:47

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