Let's say you have a planet(about the size of Earth) where the core of a planet has gone cold for whatever reason. So the planet is basically just a barren rock devoid of atmosphere or electromagnetic field.

Could you dig to the center of that planet and build a city/base there? Would the pressure of the surrounding planet crush you? Is there a limit to how deep humans could conceivably colonize underground?

  • $\begingroup$ You need to consider 2 things: the pull of gravitational force between you and core plus everything all the way up to the surface. $\endgroup$
    – user6760
    Commented Jul 24, 2020 at 11:18
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    $\begingroup$ Your last question is the only one that matters: dig deep enough to be safe from whatever surface effects are dangerous, be that radiation, cold, or velociraptors. don't dig so deep that you can't maintain a safe temperature level. $\endgroup$ Commented Jul 24, 2020 at 12:46
  • $\begingroup$ @CarlWitthoft You're not going to be able to dig yourself away from super-intelligent velociraptors. $\endgroup$
    – DKNguyen
    Commented Jul 27, 2020 at 22:55
  • $\begingroup$ The solar wind would strip the atmosphere of a planet with a cold core - such as Mars. I don't think it wise to trap yourself in the gravity well of a dead planet. $\endgroup$
    – Tangurena
    Commented Jul 28, 2020 at 22:25

2 Answers 2



Reason 1: Even here on Earth there is a problem with mines once they get down to about 7000 feet or so: Creep. The rock slowly deforms under the pressure. Maintaining a tunnel requires ongoing maintenance reaming out the slow flow of rock that tries to squeeze the opening shut.

Reason 2: Useful energy requires a source and a sink. The source has to be at a higher temperature than the sink. Example: We can burn coal to create steam drive a turbine to make electricity. As a heat engine this is high temperature coal fire to low temperature wet steam with some of the difference extracted.

The coal in turn came from the difference between high temperature sun, and low temperature space.

So where does your colony get it's energy.

That said there are MANY stories (See Poul Anderson's collection, "Tales of the Flying Mountains" about asteroid mining. Energy is external to the asteroids, often solar. Colonizing the surface, or just under of an asteroid makes all kinds of economic sense -- ready source of metals, of oxides, of mass in general. Protection from radiation. But the core of a planet? You need a compelling reason to put up with the immense difficulties.

Comment about lack of structural integrity of asteroids:

Mass+ energy = habitats.

Take a ton of plastic that has a plasticiser that will boil off. Stick a hose in it, and inject ton of gas. Doesn't matter much which. Inflate the plastic until it's a bubble 10 microns thick.

Glue a door to it.

Enter the bubble, move to the centre and boil a pound of aluminum on one side of a large plate. Half the aluminum freezes to the plate, the other half coats half the interior of the bubble. Vapour deposition may not work. There is still some very thin gas in the bubble. Electrostatic?

Anyway now you have a several hundred foot diameter bubble that is silvered on one half. Makes a badly focused mirror. Wanna bet that the focus isn't hot enough to melt most rocks?

Make other bubbles. Put medium sized asteroids in them. Put enough mirrors shining on the rock to boil out the gasses. Collect and store for use later. I'd expect the gas composition to be similar to cometary gasses.

Spin a larger asteroid slowly. Fuse the outer 10 meters of crust into something more solid and durable. Now hollow out the inside.

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    $\begingroup$ Though the hollow asteroid stories mostly date from before we had any knowledge of their structure. Nowadays the smaller ones, at least, seem to be more like loosely-aggregated "rubble piles" than something you could dig tunnels in. $\endgroup$
    – jamesqf
    Commented Jul 24, 2020 at 3:53
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    $\begingroup$ The biggest of the asteroids, Ceres, is basically a big ball of mud, with the center being perhaps more accurately described as waterlogged mudstone. Probably quite easy to dig through near the surface, but not much better for deep tunneling. However, it's small enough and its gravity low enough that it might be technically feasible. (Desirable is another matter.) $\endgroup$ Commented Jul 24, 2020 at 23:48
  • $\begingroup$ Good points. But it's still mass. $\endgroup$ Commented Jul 27, 2020 at 19:35

Why would one want to reach the core of a planet?

To reach it one has to dig a few thousands kilometers of rock, and there would be only dense elements, iron and nickel in the case of Earth, which could be found also more superficial layers. And the more the hole is dug deep, the more gravity is pushing to close it.

I remember reading in the 90s about the Kola deep drill, where they said that samples of rocks taken at the depth of "just" 12 km would explode on their own when taken, just for the sudden release of the pressure.

Here you even have a cold core, so you are not even chasing for the last source of energy in a dying world.

All in all I think the feat is not worth the effort.

  • $\begingroup$ The core might be electromagnetically "cold," but it can never be thermally cold $\endgroup$ Commented Jul 24, 2020 at 12:47
  • $\begingroup$ @CarlWitthoft , a planet with no dynamo action and no radioactive elements will eventually reach equilibrium with its environment. I can't say for sure what that would look like for a planet near a sun, but for a rogue planet, that would be very near absolute zero. So, yes, the core can be thermally cold. $\endgroup$
    – cowlinator
    Commented Jul 28, 2020 at 1:36

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