High-tech Ents foresee a calamity befalling their beloved and treasured green planet. Surface temperatures will rise well above 500 Celsius (arbitrary, basically just very hot) for thousands of years, before cooling back down.

They enact many preservation plans, one going towards space, one going underground. Their rocket science is very limited, but their bioengineering is at near miraculous levels. For this reason they prefer to go underground, where they have less initial limitations. (All other tech, especially material manufacturing are high level too, since nature produces some amazing materials)

They have the capability to dig deep and create huge cavern habitats with different biomes to save much of their current wildlife. They already modified numerous organisms to manage water and waste recycling, humidity, inner temperature, oxygen (air), light, etc. and they are mostly autonomous, requiring no maintenance. They will however need constant energy and produce heat.

The underground habitats will be surrounded by extreme high temperatures from below (core heat) and above (the calamity), closed off from natural sunlight (free energy).

So the Ents face 2 big problems:

  1. Even fully insulated from outside heat, all life produces heat. If they can't dump the excess heat, they will eventually cook themselves. Especially that their environment maintaining organisms produce even more than normal. Storing it in some form is not preferred for fear of the calamity lasting longer than calculated.

  2. Even with vast stockpiles, they can't store enough energy inside these habitats to last forever. They will have to generate it some way using their current environment.

These are the two main problems the Ents identified so far, and can't solve without handwavium technology. The Ents, close to nature, abhor any form of handwavium. However in order to save the many lives of their world, they are prepared to put up with the least amount of it.

Is there a reasonable scientific way to solve the problems given the circumstances? If not, what type of handwavium should the Ents invest?

The solemn Ent researchers look towards the starry sky for answers.

Edit: though it isn't the main issue, the Ents envisioned their habitat's "shell" as layers of high melting point, low heat exchanger material with close to vacuum conditions in between. They still looking into the best composites, but figured if they can dump the inner heat outside, they could do the same with any leaked in heat.

Edit 2: The "miraculous" bioengineering is their own, arrogant estimate. Basically, they can produce machines and infrastructure which take care of its own maintenance, given steady energy supply. It also helps with the long-term self-sufficient biome designs, given the 2 connected problems solved.

Edit 3: Ents - tree like plantoids, sentient, biological. Technology, culture somewhat similar to humans but more heavily emphasize co-existence and symbiosis with nature.

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    $\begingroup$ there will be a gradient of the temperature (or surface will be hotter than planet guts, or planet gut will be hotter than surface) and that is what you need for extraction of energy which you can use to cool them. There still some problems to overcome, so as specific of situation is important. Generally, a planet can be a capable heat accomulator, it is quite massive body. So as maybe they will find sources of fissile materials and their biological magic will be enough to produce reactors with direct energy to electrictiy conversion $\endgroup$ – MolbOrg Jun 22 '19 at 20:29
  • $\begingroup$ Their bioengineering is at near-miraculous levels and they can't bioengineer a solution to heat generation and dissipation? Just how miraculous is their bioengineering tech? $\endgroup$ – JBH Jun 22 '19 at 21:55
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    $\begingroup$ It turns out the lead researchers were busy designing the habitat itself, leaving the easier problems to their less gifted undergraduate students, as a test. Which they failed, until a more experienced and benevolent senior pointed them the right direction. $\endgroup$ – Lupus Jun 22 '19 at 22:05
  • $\begingroup$ This just popped up in the queue as too story-based. Please remember that story-based means the question is too dependent on the circumstances, actions, or plot of the story. Taken to its extreme, all questions are story-based. My personal test is, if you remove the story from the question, do you still have a question? In this case, you do. Solving the problem of temperature control in an underground environment can be answered without any dependency on the story at all. I don't believe this question is TSB. $\endgroup$ – JBH Jun 22 '19 at 22:12
  • $\begingroup$ Please define "ents." It's not capitalized, so it doesn't sound like a species name. If it is a species, tell us what type. Humanoid? If it's a culture, say so. Is it a robot? I have no idea. It's not a word one can Google and people shouldn't have to Google to understand your question. $\endgroup$ – Cyn says make Monica whole Jun 23 '19 at 1:38

In short, it's a tough engineering problem, but it's not a physically impossible one.

500 degrees Celsius might sounds like a lot, but it's really not that bad. Aluminum melts at 659 C, and steel at 1371. That means we can use heat pumps to concentrate the heat from the habitat and raise it to a temperature higher than the 500 degree exterior, which means we can reject it to the exterior. This process will require energy, which will generate more heat, which will require more energy, but calculating the exact optimum point where your heat rejected is maximized compared to your heat generated depends on a slew of technical assumptions and I'll leave it as an exercise to the reader.

Now, a few suggestions:

-Seasponges actually have an internal structure somewhat akin to aerogel, which is an excellent insulator, so biomimetic aerogel insulation is something you might look into instead of vacuum chambers which are more annoying to work with from a technical standpoint.

-Geothermal energy might not be the best bet. It depends on the location, if there's a nearby source of geothermal energy, that might work, but otherwise it might be better to use nuclear, it's a lot more energy dense so you can store enough fuel for thousands of years of operation, and you're not limited by geography on where you can put your bunkers.

-Finally, you might not even need underground bunkers, the main use of them would be to protect you from the high surface temperatures, but if the planet's top layer is also heating up, you'd be better served not dealing with the hassle of digging out these bunkers and instead just make a bunch of insulated spherical habitats on the surface.

  • $\begingroup$ Saw a somewhat similar mechanism in a game called Oxygen Not Included. The game didn't have all the physics right, so wasn't sure if it would work in real life. Thanks for going out of your way for the extra suggestions. $\endgroup$ – Lupus Jun 24 '19 at 22:27
  • $\begingroup$ instead just make a bunch of insulated spherical habitats on the surface. – This does not sound like a good idea due to air being able to freely move and transport heat in the atmosphere as well as not blocking thermal radiation very well. Also, if the reason of the surplus heat is increased sunlight or similar, you will have to somewhat deal with that constant energy influx. $\endgroup$ – Wrzlprmft Jun 25 '19 at 9:09
  • $\begingroup$ The original post specifies that the habitats are surrounded by a high heat environment, I took this to mean they are submerged in a hot reservoir with effectively infinite heat capacity. In this case, the surrounding environment will only have a small effect on heat transfer, as the vacuum (or aerogel) walls will contribute far more to the total resistance to conduction. Direct solar radiation can be easily dealt with by the erection of a reflective shade over the habitats. $\endgroup$ – Algebraist Jun 26 '19 at 3:04

To build on Algebraist's answer, I would use thorium rather than conventional nuclear reactors. Depending on where you build the bunkers, you could collect the thorium while you dig, and you wouldn't have to deal with all the radioactive waste once the nuclear fuel is spent. And even if the reactor would spend most/all the fuel, you would have to maintain the complicated safety systems that conventional reactors require

  • $\begingroup$ Thanks, will read up on thorium reactors. Sounds interesting. $\endgroup$ – Lupus Jun 24 '19 at 22:28

The power required for cooling will be relatively significant, but not enough to make things wholly impossible. Although thermodynamics tells us that we must expend more power on cooling than we use in the habitat and this could limit things.

Coefficient of performance for air conditioning is COP= T_Cold/(T_Hot-T-Cold), so assuming the average temperature inside is 20 C, the maximum coefficient of performance we can get is 0.61. This presents a predicament because it means that we must spend at the very least 1.6 the energy we expend in the habitat on cooling. So maximizing efficiency is of utmost importance here. I'm going to arbitrarily consider a spherical habitat 110 meters in diameter with an arbitrary 1 meter thick layer of insulation. We choose a sphere for ease of analysis and also because spheres are the best at minimizing surface area as we increase volume. So going from one of NASA's studies on making a rover for venus with a cooled interior for electronics, we'll use multilayer insulation. This might give us the most bang for our buck and may even outperform aerogel at these temperatures. Assuming a temperature difference of 480 K between the layers and that we have near vacuum, plugging into a handy study NASA did on multilayer insulation we find that a thermal conductivity of 0.02 W/(m*K) is reasonable to assume.

So the heat transfer through an spherical shell with inner radius r1, outer radius r2, inner temperature Ts1(20 C) and outer temperature Ts2(500C), and thermal conductivity is given by Q=4*pik(Ts1-Ts2)/((1/r1)-(1/r2)). Plugging in the temperatures(in kelvin) and radii we find that we need to get rid of ~37 KW of heat. Assuming we get a coefficient of performance of half of the thermodynamically ideal coefficient of performance we find that we need to expend 37KW/0.305=122 KW to remove this heat. This is a fair amount of power, but not utterly unreasonable. As we increase the diameter of our spherical habitat, the ratio of heat we have to remove relative to the cooled volume we have decreases.

Power generation becomes a bit more difficult as we must reject heat to a 500 C environment. The inside of our heat engine needs to be much hotter than the environment in order to be efficient. We could potentially use a very high temperature reactor for power, where it's possible to achieve outlet temperatures of 1000 C. Another interesting option is a fission fragment reactor, where we can avoid heat engines entirely and harness fission almost directly. There's always fusion.

A challenge is that it seems you are using bioengineering for everything. There aren't many organic materials which are stable at 500 C and above. Getting biology to produce metals and ceramics which are stable at these temperatures may not turn out to be possible.


Going underground has the advantage of insulation for relative cheap. And with a large degree of robustness. Fifty meters of rock is going to be very good at insulating, and also very resistant to minor things like weather. You'd need to have a good knowledge of the geology and earthquake frequency and such to be confident of the vault. But that could certainly be approached with some fairly easy things. If there are million year old caves nearby, probably the geology is OK, just as an example.

But after 1000's of years they will need to have some kind of cooling. How well they can manage that depends on how much energy they can generate. If they are OK with nuclear power, for example, they could run some honking-big A/C units. The exhaust side, on the outside of the habitat, needs to be hotter than 500°C in order to lose heat. So possibly the working fluid is interesting to choose. Maybe the A/C units need to be staged to be sufficiently efficient. One stage moves heat from the habitat to an intermediate step, using more ordinary A/C units. Another stage moves heat from there to outside using some yet-to-be-designed A/C units with some interesting new design. Off hand I don't know what fluid to select to work in the range of 500°C, but it's likely something could be found.

Nukes operating with a "cold" side at 500°C are possible, but they'd be a challenge. The working heat transport fluid might be lead. There are designs on the blackboard now for reactors that use molten lead as coolant. It's conceivable they could have most of the actual reactor completely outside the habitat in order to keep the heat loss problem as small as possible. Only the control systems and computers need to be inside.

Nukes will also help them generate light. I presume ents will want light, probably with a spectrum as similar to sun light as reasonably achievable. Some good LEDs will be useful there, efficiently producing light while producing minimal heat. Pick them to produce the right spectrum and the ents will be less depressed at being underground.

They might need to develop various tech to go with. For example, they might need to make brief trips outside to maintain equipment, take observations, obtain raw materials, etc. So they would need insulated suits with air and cooling. Possibly tanks of liquid air would cover both.

  • $\begingroup$ Thank you, will read up on it. $\endgroup$ – Lupus Jun 24 '19 at 22:38

Instead of making a bunch of extra heat by piping in geothermal energy or running nuclear power plants, just create a self-contained Power/Heat Cycle.

Plants already sequester thermal energy into chemical bonds; so, if I were to try to solve this problem thinking like a plant person, I would try to find a way to turn my heat into yummy starches. The creation of these complex organic molecules is an endothermic reaction which would sequester your heat into chemical compounds cooling your caves. Done right, these chemical compounds could then be processed into biodiesel that is burned to power the lights and other technology while releasing your CO2 back into the air to be reused. Burning the fuel would in turn heat up your cave system feeding your heat sequestering plants all over again.

By keeping your own thermal/power economy balanced using local reserves, the only heat you'd have to worry about would be from the outside, but your heat based economy would help you here too. As you're cave heats up beyond what it should be, your sequestering plants would go into a bloom, reproducing and growing faster as the additional heat feeds them. This would mean extra fuel to power the fancy heat exchangers as discussed in some of the other answers here.

Normally a self-contained Power/Heat system like this does not work in the real world because your heat is always radiating off to somewhere colder, but in this case being sandwiched between two heat sources is what makes it sustainable.

  • $\begingroup$ (Ok, reminder to self. Don't try to type the degree symbol in a comment.) I don't agree. Plants need an energy source to regulate temperature, just as other organisms. Forest canopy, for example, tends to keep a nearly constant temperature underneath. This is done by controlling shade, evaporation, and some other things. But that all requires energy. Plants can't convert local ambient temperature to sugar. This would violate the 2nd law of therm. You need an energy source, and that can't be 100% efficient. $\endgroup$ – puppetsock Jun 26 '19 at 20:22
  1. Phase change of water. A great way to soak up heat is via phase change. The ents are aware that the crust contains vast stores of liquid water. They vent that water to the surface, accumulating enormous quantities of it. When the heat come the water acts as a buffer, absorbing the heat as it changes from liquid to gas. The surface water also buffers the underground by virtue of its great heat capacity.

Moving and evaporating water seems like good nature tech.

  1. Iron fusion. This is not terrestrial magic as much as sidereal magic. Fusion of elements below iron releases energy and fuels stars. Fusion of iron or elements higher in the periodic table absorbs energy and kills stars.


Higher mass stars will switch from helium to carbon burning and extend their lifetimes. Even higher mass stars will burn neon after carbon is used up. However, once iron is reached, fusion is halted since iron is so tightly bound that no energy can be extracted by fusion. Iron can fuse, but it absorbs energy in the process and the core temperature drops.

This process is why stars die: they are cooled to death as fusion of iron drinks heat and changes it to matter. If this process can cool a star it can cool your world, and make it a little heavier. Alright arrogant tech wizards: saddle up!

  • $\begingroup$ Interesting. Knew about iron fusion, but never even considered it. Have so much to read up on! $\endgroup$ – Lupus Jun 25 '19 at 22:39

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