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The Question of the Tunnel through a Planet grinds my mind.

How could you dig a Hole through the fluid mantle of a planet?

Would your walls not constantly break and collapse on you after the 15-20 km of the crust?

Would it be a straight, spiraled one or something totally different?

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  • $\begingroup$ The Earth's mantle is not fluid. The Earth's inner core is fluid. $\endgroup$
    – Oldcat
    Dec 5, 2014 at 21:24

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You have several problems here, let's assume the logistical ones are all solved (as they already are for long distance tunnels) and just focus on the engineering ones.

You have six problems with a tunnel through the magma:

  • Making the hole
  • Temperature
  • Pressure
  • Leaks
  • Seismic activity
  • Currents

Making the hole

Actually making the hole is easier in the liquid area, assuming you can exert sufficient force you can just push the liquid mantle out of the way and it will flow. The problem is stopping it from filling back in again.

Temperature

The mantle has a temperature ranging from 500°C to 4000°C. (Some research suggests the core may even reach 6000°C). Even Tungsten melts at those temperatures. Diamond will melt at around 4000°C so might survive if you made the tunnel out of pure Diamond, but that's at atmospheric pressure. Increase the pressure and even Diamond melts before you reach the core, bringing us on to our next problem.

Pressure

You are talking 140GPa (Gigapascals) of pressure. That's 10,000,000 times atmospheric pressure. The walls of the tunnel would need to be incredibly strong to withstand the pressure, or the interior of the tunnel would need to be pressurized to the same level which would make travel through the tunnel problematic to say the least.

Leaks

At those temperatures and pressures even a tiny leak would quickly become catastrophic and you could well lose the entire tunnel.

Seismic activity

The tunnel would be vulnerable to shocks and quakes in the upper areas before it reaches the mantle itself. These forces would act to deform, twist, or even sheer straight through the tunnel.

Currents

The mantle is flowing, the lava is moving. This movement would try to carry your tunnel along with it. That would provide a lot of stress into the already stressed material and again would cause the tunnel to flex and move as the mantle flowed around it.

Conclusion

It's not possible with any current scientific ability to dig a tunnel through an earth-like planet. A smaller planet with a cooler core would be a different matter.

The materials science needed is incredible, and most likely physically impossible. The development of force fields and other non-material reinforcement might make it possible in the far future but such concepts are speculative at best.

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    $\begingroup$ In Dwarf Fortress when we want to get past an aquifer (effectively a water layer), we place pumps around a hole and remove the water fast enough to make a clearing for scaffolding (lol). When people get down to the magma layer, they usually dig through it using pre-existing nerves/pillars that form through it (I assume real mantles are all fluid). What could work is cooling the magma with water into obsidian and digging through that (in-game). This idea was in my head when I read this question. Wouldn't the water flowing in cool down the magma slowly to form a path, assuming supports? $\endgroup$
    – mechalynx
    Oct 1, 2014 at 11:36
  • $\begingroup$ It's an interesting idea, if you cooled the magma enough it would naturally form a pillar of stone around the hole and help support it. However cooling down that amount of rock from that high a temperature while surrounded by more rock also at that temperature....that again would be a massive engineering challenge in its own right. $\endgroup$
    – Tim B
    Oct 1, 2014 at 12:03
  • $\begingroup$ It might not be as simple as just letting the water drool over the magma (although reality can be surprising in how easy some things are) but it might be possible to cut out a disc and then sink a container that will hold sea-water (through pressure and gravity), and have the container disintegrate in a controlled manner so the water is most effective in cooling the magma. I'm thinking exposing the magma to the water in rings, top to bottom and each ring being removed slowly across its circumference. That way, you can avoid the effects of the surrounding magma, to an extent. I think :P $\endgroup$
    – mechalynx
    Oct 1, 2014 at 12:11
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    $\begingroup$ You are at real risk of getting a en.wikipedia.org/wiki/Phreatic_eruption - the explosion of Krakatoa caused Tsunami measured around the world and killed 36000 people. That was surface magma at "only" 1000 degrees hitting water.... $\endgroup$
    – Tim B
    Oct 1, 2014 at 12:20
  • $\begingroup$ Um, oops? :P Not to diminish this but, from the image it seems as if the magma was already erupting and had created a conduit to the surface before water came in contact (although I could be wrong of course). Explosions are more violent when they have higher pressure compared to their environment - would this be as violent at the bottom of an ocean? Also, a work-around might be filling/lining the container with some cold but less volatile material, perhaps liquid oxygen or nitrogen (I'm assuming they're less volatile when heated but not sure). I think the real problem is Leidenfrost layers. $\endgroup$
    – mechalynx
    Oct 1, 2014 at 12:40
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I fear that conventional science may fail you here. It is not technically possible, for instance, to actually "drill" through a liquid, for instance. (To say nothing of the incredible heat and pressure encountered in this scenario.)

However, this is the Worldbuilding SE not the Geophysics SE. Which means if we need to "drill" through the mantle of a planet, we just need to make it plausible in-universe.

I'm going to assume that, for this, you are in a generic soft science fiction universe. This gives you some options:

Force Fields

Generated and focused walls of force could be used to "simply" push the molten rock and metal out of a path through the mantle. A setup like this would likely involve a series of generators down the center of the cavity to reinforce the tunnel. This process should take incredible amounts of energy to make it plausible. (Whether that is done as a trivial matter by an ultra-tech civilization or as a titanic undertaking by lesser civilization is up to the author/designer) A tunnel like this might actually flex in order to resist any currents in the molten sea of the mantle.

Magma Ships

If the goal is simply to travel through and/or harvest the liquid material of the mantle, an advanced vehicle (properly shielded) could "swim" through the liquid like it was an ocean. This solution would still need a hole drilled through the crust in order to deploy these craft.

I could go on really, the possibilities are endless. The important thing here is to know the problems with what you're trying to do, and then how to overcome those issues in a reasonable manner.

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Like Tim B, I will assume that logistics are solved, and deal purely with the walls of the tunnel.

Supposing that the walls are made of diamond and it is available in practically unlimited quantities. Diamond begins to transition from molten liquid form to forming solid chunks at 11 million atmospheres (http://news.discovery.com/space/alien-life-exoplanets/diamond-oceans-jupiter-uranus1.htm).

Suppose that the tunnel walls were to be a thickness of diamond such that at least temporarily a solid inner core could be maintained while the outer edges of the walls melt. For example, 100 metres thickness of diamond. We now "merely" need the solution to two issues:

  1. Cooling of the tunnel: We have already assumed that logistics are solved, so this is merely a matter of large scale cooling or perhaps a new geothermal power solution.
  2. Loss of diamond to melting: The more challenging issue. Diamond from the outer wall will disperse in the molten core material. Diamond will have to be continuously synthesized at the inner wall and slowly migrate to the outer wall. As crazy as that might sound, living organisms create similar structures through a continuous cycle of interior growth and exterior loss.
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A few out-there ideas, sci-fi concepts included:

Spinning the planet, along its axis: Although only possible pole to pole, spin the planet fast enough to reduce pressure - although this would only work really by destroying most of the planet.

A wonder material made by nanobots, and designed by AI: We may not know the answer, but put a good smart AI onto the task and it may yet find a solution, even a brute force one such as using untold nanobots to continually mine, cool, and create support structures, using a wonder-material.

Wormhole: Although wormholes are theorised to exist, but are extremely small and last only for fractions of time, there may yet be technology to be invented that allows for longer lasting, larger wormholes. This could bypass the magma completely.

Anti-gravity: If someone can create a repelling force, pushing away heavy objects, it may be possible to maintain a hole using many repelling anti-gravity generators. Again, to-be-invented.

All-in-all, pretty hard stuff to invent yet. Makes going around the planet that much more attractive.

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Drop a small black hole at your feet. If it falls straight, it will fall through to the center of the earth and go all the way (almost to the other side of the world) and then fall back to the center of the earth and will repeat this many times until it settles at the center. I'm ignoring the rotation of the earth which will complicate things.

The black hole will gobble up magma and leave a tunnel behind. The tunnel will unfortunately quickly refill. Of course, the filling tunnel is the least of your problems. It won't take long for the whole earth to be gobbled up.

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  • $\begingroup$ The black hole might also very well make a big kaboom long before it has time to do much of anything else. But of course, there's nothing magical about them. $\endgroup$
    – user
    Nov 25, 2015 at 15:45
  • $\begingroup$ Unfortunately, neither of those two things is likely. Even a black hole large enough to be more than a fraction of an nano-meter will emit enough radiation to vaporize any matter that comers close, forming a layer of low density plasma around the event horizon. So on top of the black hole being really small to the point where it won't carve a large hunk of matter out of the medium it's moving through, its intake will be further limited by the fact that it's actively blowing away any matter that tries to come close with a high radiation flux. $\endgroup$
    – Algebraist
    Jun 23, 2019 at 3:04

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