Welcome to my secret underground lair!

Part of my series on countering misconceptions in space journalism.

While I’ve amused myself by taking potshots at popular visions for space cities including domes, modular space stations, and O’Neill cylinders, I realized embarrassingly recently that I’d missed yet another terrific example! Tunnels! And secret underground lairs of all kinds.

InnSpace Mars city design

Of course, the usual disclaimers still apply. I’m a housebound socially isolated COVID-fearing glorified recovering physicist who claims to be able to write software, inelegantly chuntering opinions into the internet’s screaming maelstrom. Isn’t there something more useful I could be doing with my time?

More seriously, my purpose here is not to criticize but to illuminate. To ask interesting questions, as a way of motivating collective inquiry into topics of mutual interest. We are all wrong, to varying degrees. The only useful meta question is how may we go about becoming less wrong?

Tunnels are a staple of both science fiction and popular journalism regarding human habitations on the Moon, Mars, or other rocky places. They’re fun to write about and interesting to put on screen. I’ve lost count of the times I’ve seen beautifully illustrated Mars city maps featuring a hexagonal grid of domes connected by tunnels. On a visual level, it certainly ticks all the right boxes.

And yet, while I’ve wasted years of my life on real estate websites I’ve never seen a subterranean house on the market. They do exist, if you want a converted ICBM bunker or limestone cave, but they’re a definite rarity.

Why?

The simplest explanation is that digging holes, particularly really deep ones, is very energetically intensive and expensive. The cost of building a road tunnel works out to be about $100,000 per meter, or equivalent to a stack of Hamiltons of the same length! For comparison, $100,000 will buy materials and labor on a respectable manufactured home, or substantial renovations.

Indeed, on Earth, underground construction is basically unknown except for nuclear bunkers. These have two powerful reasons to accept the cost and inconvenience: unlimited sweet DoD money, and surviving really big explosions.

Why build underground in space? The usual explanation is to provide shielding against galactic cosmic rays, or micrometeorites.

It is true that tunnels deep underground are relatively safe from both, and also well thermally insulated. But as I discussed in the blog on space radiation, relatively little shielding is necessary even in areas that people spend a lot of time, such as sleeping areas. And even if that works out to be a meter or two of rock, it’s orders of magnitude less effort to drop sandbags on the roof of some structure constructed on the surface, than to dig a hole of the necessary size deep underground.

Micrometeorites are not a concern on Mars, which has a thin atmosphere, and can be well shielded on the Moon with a thin blanket of loose rubble.

If there’s a central point to my blogs on space architecture, it’s that our cities and houses on Mars will look and feel a lot more like regular houses on Earth, and for the same reasons. It may not be very exciting, but the most important consideration for design and construction, on Earth or in space, is expedience. Given the relative scarcity of human labor in space cities, structures will have to maximize usable area and minimize effort even more than on Earth. Instead of tunnels, think warehouses and aircraft hangars! At least they can have natural light.

33 thoughts on “Welcome to my secret underground lair!”

    1. The problem with lava tubes is that they’re probably not the right size and in the right place. They’re hard to expand. They’re not strictly stable. They’re full of cracks so hard to seal. They might contain interesting things but I wouldn’t want to live in one.

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  2. The appeal of tunnels is that you don’t need to build a roof to pile stuff onto, nor do you need to have sandbags or whatever in order to pile that stuff onto that roof. However, the sort of tunnel boring machine suitable for constructing these holes in rock would be … well, it would be a significant investment! I think you’d want some sort of atomic powered machine which melts the walls (for an airtight sleeve), but it’s unclear exactly how best to remove the material within the tunnel.

    But if we were really serious about this idea, I think we’d do better looking at the Martian polar ice caps. Melting your way through ice is a lot easier than melting your way through rock. You could have a simple steam heated pipe helix its way through the ice, melting out blocks of ice which easily slide back out the tunnel. At the end of the tunnel, you need to remove this waste ice, of course, but this is a comparatively simple task than setting up a rock conveyor belt etc.

    Note that ice can make a robust compression structure, but not a robust tension structure. Therefore, you want to tunnel at a depth where the weight of the ice above it exceeds the desired atmospheric pressure. This will be roughly 3 times deeper than what’s needed for long term radiation shielding, but that’s okay.

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    1. Check out Project Iceworm to investigate the challenges of tunneling in ice. I know it was done in Green Mars, just as melty tunnel boring was done in Asimov’s “The Gods Themselves” but both have significant drawbacks.

      First, ice is ductile and flows. So the tunnels would not have stable shapes. Also, the poles are cold and have much less light (especially in winter), making solar power tricky.

      Tunnel boring machines (TBMs) generally bore a hole by breaking the rock into big chunks and removing it on a conveyor. The smaller the chunks, the more area of fractures are required, which increases the energy requirement necessary to break those chemical bonds. Melting requires breaking EVERY chemical bond, not just every billionth or so, which is a severe energy problem.

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      1. You misinterpret my comment – I suggest only melting the ice into big chunks by spiraling a helix through it with a steam heated pipe. The resulting ice chunks can then slide back out the tunnel – no conveyor belt needed thanks to low friction.

        The volume of ice which is actually melted is small.

        The lack of solar power is not such a big deal, because there is no location on Mars immune to prolonged dust storms. While an unmanned probe can shutdown to minimal power in such a circumstance, human beings require life support regardless. As such, all locations of Mars will require nuclear power anyway.

        As for the problems caused by flowing ice – well, we won’t know how severe a problem it would be without further research. The goals and scale of a Martian station would be different from what Project Iceworm was attempting, and the lower gravity of Mars would mitigate against collapse. Importantly, Project Iceworm was greatly under-pressured compared to the surrounding ice, which is why there were forces tending to narrow and ultimately collapse the tunnels. In contrast, Martian ice tunnels would be neutral pressure compared to the surrounding ice. There would be little force tending to narrow or expand the tunnels, nor a strong force trying to collapse the tunnel. (You’d want it to be a bit under-pressured rather than a bit over-pressured because ice is more robust under compression than tension.)

        Considering the great differences between an under-pressured tunnel vs a neutral pressured tunnel, I wouldn’t be surprised if these tunnels would be stable over multi-decade scales. That would be good enough, considering new tunnels would be excavated continuously.

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  3. As on earth, wouldn’t cut and cover be far more efficient than tunneling? You get the advantages of thermal massing and stable temps (not warm sadly) with the option for both earthen roofs (industrial) and pressurized canopies (for humans and ants). Hot countries often built into the earth (not under) for this very reason, and the energy cost of heating on Mars will be astronomical if we just use inflated canopies. Ask the Eden Project in Cornwall (domes I know, but uses ETFE pillows as the membrane).
    A colony will need dirt as a resource (as opposed to ores) so strip mining a channel kills 2 birds with one stone.
    It’s an interesting conundrum, and I’m loving re-reading your posts with this in mind.

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    1. Cut and cover can work, but it’s even cheaper to (not) cut and cover.

      Rock is not a great insulator. It’s okay, when a lot is used. But if thermal insulation is the be-all and end-all for some transparent dome, like the Eden Project, a cleverer scheme may be required. Personally, I envision a green house with lots of thermal mass near the ground, like ponds and tanks of water, to smooth out diurnal variations.

      The main challenge with specifying that every structure needs an excavated basement or whatever, besides breaking and moving all the rock, is that one can never be sure exactly what’s down there. Frank Lloyd Wright tried to excavate a cavity at Taliesin West to build a small theater, but found that even dynamite didn’t move the rock much, and eventually built the structure mostly above ground. Indeed, while tunnels are expensive it’s worth noting that with current technology, something like 85% of candidate tunnel locations are simply not possible, for any price. The 15% of possible/easy places are responsible for the inordinate cost and difficulty. In short, excavate only what is necessary to build a foundation and then focus energies elsewhere.

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  4. Besides breathable air, livable temperatures, and a radiation-blocking magnetosphere, Earth also has a great variety of ores, and oceans to ship them on. If it were necessary to get all the minor components of every alloy by quarrying vast amounts of ordinary rock, and the rocks in one location were not all that different from those in another, we might combine quarrying with construction. I’ve gone back and forth on the question of how good a selection of ores is likely to exist on Mars, and my impression of the radiation hazard is closer to yours than to that of the typical underground-city enthusiast. But we may be off about the long-term risks from the radiation that gets through an ordinary roof there. If so, it’s not unthinkable to me that some of the prime living space would be in the quarries.

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      1. I don’t imagine tunnels as the main configuration for below-grade useful space (hereinafter BGUS). So specialized tunnel-boring machines aren’t of much interest to me. A tunnel is just a roadway underground, and roads belong above ground unless the topography and the traffic level are both very high. Mars has crazy high topography, but it will be a really long time before the traffic will reach the level necessary to justify a tunnel.

        The premise for BGUS is that we will need to excavate quite a bit of rock, whether we want to make steel or basalt fiber or whatever. Plastics and water are the only things we can get without moving rock. They’re great stuff, but they aren’t the basis for a complete supply chain by themselves.

        So we’ll be digging up rock to process it, and we won’t have an ocean to ship it on. We won’t want to haul it any farther than we have a good reason to. So the spaces we excavate will tend to be fairly compact. Not tunnels. But will they be completely underground, or merely below grade? And will they be useful spaces that we carve out as we go, or will they be mildly pesky holes that we make sure we don’t fall into, but otherwise ignore?

        On Earth, the rocks everywhere are different from the rocks everywhere else. So you want to quarry wherever the best rock for your purposes happens to be. And there’s lots of infrastructure already in place, so everything else has reasons to be wherever it happens to be, too. So quarries are inconveniently located holes that you ignore.

        On Mars, there’s a lot of rock that’s a lot like all the other rock, and the first city will be planned starting from nothing. So I think the quarrying will be integrated with the site preparation.

        And there will be a lot of unknowns. Some will be hard to do anything about, or not all that scary, or both. The effect of decades of exposure to slightly higher levels of slightly different types of radiation, though? That’s scary, and easy to do something about. I think the main living spaces will be much more heavily shielded from radiation than the risk actually justifies.

        That may mean underground, or it may mean dug and covered. I think it will depend on the difference between good steel that requires digging up and extracting various trace elements, and Martian standard steel that doesn’t. If we need really large amounts of good steel, we’ll be doing lots of quarrying, and we might as well have some of it be fully underground. If not, the amount of quarrying will be small enough to be just basic site prep.

        Hmm, I wound up not using the phrase BGUS again. Oh well.

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      2. Many good points.

        I strongly suspect that using tbms for mining will be a more versatile tool than anything other than open cut for fortuitously surface-located minerals. The reason is that the cutting volume can be heated and pressurised, so more human friendly.

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    2. The USGS created a geologic map of Mars, with several different types of soil/rocks as they appear in different areas. So it seems there is some differentiation of rocks on Mars, it’s not just one homogenous mix. I’m no geologist, so I don’t know if this indicates some areas are more likely to contain ores like we can find on Earth or not. https://pubs.usgs.gov/sim/3292/

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  5. Not directly related to the possibility of putting some living-space in the quarries, but brought to mind by it —

    Would geothermal energy be feasible on Mars? Boring deep wells is cheaper than it used to be. Natural gas prices are at historically low levels, even as the most easily accessible resources have been depleted. Pumping water through fractured hot rock might be an economically viable source of some elements, in combination with energy.

    As you’ve probably noticed, I like the idea of getting stuff in combination. If there’s a unifying theme to my vision of a Martian city, it’s diversity. Do what you need to, in order to get the scarcest stuff you can do ISRU for. Then get the stuff that comes along with it for free, or at a big enough discount, in the amount that it comes along, even though you’ll need to get the rest of it on other ways. Repeat for the next resource and the next until you have the whole supply chain — either the least scarce one as you plan production at the same stage of development, or the next scarcer one, because you’ve expanded your economies of scale. Then you have diversity of capital by age: stuff that was optimal when some things had to be shipped from Earth will still be worth using, even though it’s no longer the best way to build more of it after the range of locally produced commodities has expanded. Then there are some relatively small-volume projects: the research you can handle at each stage of development, the monumental architecture you use to feed the dreams of your support back on Earth, and so on. All in all, it makes me think that no domes at all, or no tunnels at all, is far from certain, even though I generally agree that domes and tunnels are, to put it politely, suboptimal for most purposes.

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    1. Some shallow liquid water, geothermally heated, would be awesome. Geothermal power probably there’s not enough to meet demand. On Earth 99.99% of surface heat is from the sun. Mars almost certainly similar.

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  6. I just want to say thanks and congratulations on your foresight and dedication in spending years compiling this material – just in time for a globally enforced Easter staycation. Do you work in crystal ball tech?

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  7. I think TBMS may not be the best tool in the arsenal for material excavation; there’s a daunting array of techniques underground mining. It’s hard to compare, because realistic cost estimates aren’t cheap (they cost money or time).

    Large scale excavations underground seem to vary from $5/ton to $80/ton (or $1.8-$30/m^3), which is substantially cheaper than the $100k/meter TBMS price (or $2k/m^3). Your numbers aren’t wrong, so why the discrepancy? High value locations, surface constraints, small scale, lack of profit incentive, and other factors probably conspire to inflate tunnel projects. (Dan pointed out haulage distances are less for compact mines than tunnels). Scale and labor costs are the big factors. Equipment is often amortized over the project in TBMS projects, and over 50-100 years in mining operations.

    The Eisenhower tunnel is maybe a poor example, as it is in rugged terrain and was expensive ($622 million, 138000m^3 -> $4507/m^3) but even at nearly 3km it’s a small excavation by mining standards. A sample mine cost analysis at costs.infomine.com indicates at $7/m^3 marginal cost at 5000t/day. Practical mines on Mars will need to be big to underpin a self-supporting colony though. But labor costs will inflate the price substantially, and automation will be only slowly adopted, as human ingenuity with tools is essential to address the obnoxious problems sure to arise.

    I think underground mining will win out over open pit mining due to the ability to pressurize the environment more easily. And it’s space that will be used for storage and just to get away without having to go “outside”. I suspect underground excavated space/capita will be large, unless the ores are unusually good; but you are probably right that few will want to live there.

    Doing something useful with the excavations is another problem altogether. Shield, sand fences, road grading, steel, and glass will be the first exploiters I suspect, but a Mars colony will be importing most metals from Earth until ore prospecting and large scale transportation become practical. Gravity and areology may influence mine design as well, but probably not as much as millennia of Earth experience.

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  8. Today I’m thinking that leaching will be combined with thermal storage of solar energy.

    I imagine that energy will be a major focus. Everything I imagine happening seems to require more energy than can readily be provided by systems of reasonable mass to bring along. They’ll need a *lot* of energy to un-burn CO2, move rock, smelt iron, hydrolyze water, and so on. Some photovoltaic panels will come along, ready to be deployed. Some thin-film photovoltaic sheet will come along, to be combined with locally produced backing. But ISRU seems (without trying to calculate anything) as though it has to be too energy-hungry for that to suffice.

    So I imagine that mirrors, thin sheets of aluminum-coated plastic, with be among the commodities produced early in large amounts. By un-burning some CO2 from the air, and extracting some aluminum from ordinary rock, I imagine that the newly arrived Martians will be able to make it by the square kilometer with no imported materials and not too much local labor.

    I also imagine that pipe will be readily fabricated with almost no local labor, from crushed rock (or crushed slag, or crushed solids from dried leachate) bound with low-grade plastic.

    So the geometry I’m imagining for the quarries is compact, but strip mined. They start by digging a trench. They’re going to cover it to be able to pressurize, but they don’t pressurize yet. The rock they dump on top to hold down the membrane against the pressure is material they have to move but don’t get to use. The Martian equivalent of overburden, even though it’s the same stuff as what they are going to use. So they dig deep, to get a lot of volume of usable material for each cubic meter of rock that just holds down the membrane. Then they strip mine: as they dig out rock from one side of the trench, they dump it on the other, burying a network of pipe as they go. After they have a lot of volume crushed and piped, they run water through it at various temperatures, until they’ve leached out everything but silica and alumina, extracting and separating without ever having to move solids again.

    So I’ve changed my guess about combining quarrying with site prep.

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  9. Well, since we’re on another planet, and we probably don’t have to file any environmental impact statements why not dust off the plans from Project Plowshare

    https://en.wikipedia.org/wiki/Project_Plowshare

    and use shaped nuclear charges to dig tunnels?

    https://en.wikipedia.org/wiki/Nuclear_shaped_charge

    Of course there are a myriad of technical and legal issues to deal with, such as whether or not this would violate the 1967 Outer Space Treaty, which prohibits placing weapons of mass destruction in space. But when it comes to digging large holes in the ground nuclear weapons deliver a lot of bang for the buck.

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  10. After watching Secrets of the Underground on Discovery, I realized that underground spaces are more common than we think. Nuclear bunkers, sure, but also many other military bunkers; massive salt mines, some dating back millenia; subway systems; water supply tunnels (ancient Rome and Istanbul, avoiding evaporation in deserts like Yemen and Peru, etc); mail/package delivery to avoid crowded streets in places like London and Chicago; occasionally houses, hermitages for monks, or even whole cities (like Derinkuyu).

    A lot of it requires soft rock, as many of these were done with hand tools in pre-industrial times.

    I wouldn’t say a Mars colony would be only underground or only above ground, I think some mix is the most likely scenario. We won’t waste any space that we’ve put work into.

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  11. One place I’ve seen underground dwellings even for private homes in modern times is Coober Pedy, Australia. When visiting, I was told that the reasons are that (1) digging equipment is ubiquitous, since the primary industry in the area is opal mining, often by individuals or families, so it is cheap and easy to get someone to come over and dig your house, and (2) the climate is extreme enough (very hot in their summer) that it actually pays off in terms of saving money on climate control.

    That could be a reasonable analogy to Mars, if you assume large digging equipment will be brought for other purposes like ISRU.

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