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In many modern movies, books, articles, blogs, renders etc which talk about colonies on Mars or the Moon, I often see things like habitation modules or 3D/SDL printed regolith structures and similar surface based designs, but I don't come across tunnels or underground structures very often. Why is this?

In my opinion a tunnel would do a much better job of providing radiation shielding, temperature stability, micro-meteorite protection and provide a structure to press against when pressurizing various containers so you might need less material to line the inside. Additionally, having a tunnel boring machine could allow for potentially indefinite expansion of such a base. So is there something unpractical about tunnelling on interplanetary bodies which I'm missing (like we simply don't know enough about the geology) or why is it not more seriously considered?

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    $\begingroup$ A tunnel makes a poor graphic for an article. But, consider that the mass of a tunnel boring machine probably makes an awful lot of surface habitat, and you need a lot of surface habitat to support the initial operations of a tunnel boring machine. $\endgroup$
    – Jon Custer
    Apr 14 at 14:02
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    $\begingroup$ I agree that eventually there would be large heavy machinery capable of making tunnels. But the bootstrap to get there will be a while... $\endgroup$
    – Jon Custer
    Apr 14 at 14:05
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    $\begingroup$ Lunar lava tubes (natural tunnels) are a high contender for lunar settlement locations. en.wikipedia.org/wiki/Lunar_lava_tube $\endgroup$
    – mdc
    Apr 14 at 15:32
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    $\begingroup$ Digging tunnels through rock is hard, and the difficulty and quality of the result depends hugely on the composition and quality of the rock you're digging through, which isn't always evident before you've attempted to dig through it. What can a tunnel get you that you can't get by just burying a structure in regolith? $\endgroup$
    – Christopher James Huff
    Apr 14 at 15:39
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    $\begingroup$ I don't think it is an accident that Elon Musk owns a tunnel-building company. In order to colonize Mars, you need a fully and rapidly reusable super-heavy lift launch system, vehicles to explore the surface, solar panels, batteries, and food, among other things. Elon Musk happens to own a company which does (1), another company which does (2), (3), and (4), and his brother owns a company which does containerized, self-contained, autonomous, hydroponic gardens. I would not be surprised if the Boring Company were part of a master plan. $\endgroup$
    – Jörg W Mittag
    Apr 14 at 16:14

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One of the important factors in regard to not considering tunnels as a first colonization step:

Our modern boring technology stack uses a great deal of water.

A liquid water.

This requires having the water in the first place and then pressurizing the environment in order to keep it liquid.

Recycling this water outside of the natural water cycle will be hard. A lot of this water could not be efficiently recycled because it is forever lost to the surrounding rock.

Another liquid substance could do as well, in some cases we use e.g. diesel fuel instead of water. But water is probably the nearest checkpoint anyway.

In short, before starting any tunnel-making one will need a lot of industrial infrastructure already up and running and a lot of various stockpile.

Another important factor:

No one starts boring, digging or mining big projects without knowing a lot about the expected geology.

On Earth, we have a lot of knowledge what to expect a meter, or 10m, or 100m underground, only looking at the surface. If we are in doubt, we drill for samples. And some projects still get in trouble or fail because we run over something unexpected down under. A rock too hard to bore, a rock too weak to bear the required load, etc, etc...

Moon and Mars are quite different and we don't even know the proper questions to ask ourselves before starting to dig.

And then, the tunnel casing.

It's not only the boring. We need a lot of steel and concrete to keep the tunnel from collapsing.

On Earth, these are by far the two cheapest materials (spare for the rock already lying around) used in construction. This is because we have giant factories producing steel and cement from readily-available natural resources.

Hint: both the iron ores and the coal (and the oxygen in the atmosphere as well) used to produce steel are artifacts of billions of years of life on Earth.

And don't even get me started on what the cement is made of.

On Moon or Mars, we may find it is cheaper to produce e.g. aluminum (by some yet to be invented modification of the basic electrolysis process, using solar energy) instead of steel and concrete.

All this sums up that failing to make use of some natural underground structure (lunar lava tubes, probably martian water-made caves) the best use of the expensive rocket-carried materials will be from the ground up and not down.

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    $\begingroup$ Electrolysis of regolith is apparently being investigated for oxygen production, but it could also be a useful source of metals for construction. $\endgroup$
    – Cadence
    Apr 14 at 21:00
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    $\begingroup$ @CharlesStaats The first cement used here on Earth was volcanic ash - similar substances are quite likely to be found on both Moon and Mars. One still has to get a great deal of water. In one way or another, there will be a self-sustaining industry there. It is the bootstrapping process that we talk about. $\endgroup$
    – fraxinus
    Apr 14 at 21:32
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    $\begingroup$ The only reason water is used during tunnel boring operations is to reduce dust. It's an OH&S measure to prevent silicosis. Away from the Earth, would potential tunnel boring personnel be wearing enclosed space suits, or be working in an air filled, pressurized advancing tunnel? Lining a tunnel with steel & concrete occurs in civil tunnels. The vast majority of mine tunnels are not lined. Generally they will have rock bolts & sometimes cable bolts to reinforce the tunnel. Steel set a rarely used for ground support because of expense. In addition to rock bolts ... $\endgroup$
    – Fred
    Apr 15 at 5:08
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    $\begingroup$ Great answer from @fraxinus. I'm aware that water is greatly used to reduce dust, but it's also used to carry away slurry and (depending on the specific boring head) to maintain or equally distribute hydrostatic pressure against the rock. Here is an article about tunneling by using nuclear melting which might address some of the issues you mentioned a nuclear powered melt tunneling concept for high-speed lunar subsurface transportation tunnels $\endgroup$
    – Oom_Ben
    Apr 15 at 9:06
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    $\begingroup$ Ah, the iron ores we find most easily usable for metalworking is iron-oxide, which requires the oxygen to form, otherwise it takes absurdly high temperatures to melt. I gotcha now. $\endgroup$
    – Jamin Grey
    Apr 15 at 18:09
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Underground Habs just aren't very sexy

At this moment in time, the overwhelming majority of Mars or Moon base "designs" that one can find online are not particularly grounded in reality. Many are just concepts that people have thought up for fun, were made by architecture students that wanted to flex a bit, or were explicitly designed with the goal of telling a specific story (Movies, TV, etc).

This makes underground structures unpopular in flashy renderings. They don't look as cool as the futuristic 3d-printed buildings, and when "space fans" people want to communicate their Mars or Moon base, geodesic glass domes are far cooler than telling the audience "well, optimally the people will live in vast subsurface warrens like moles".

"Realistic" plans are few and far between, and the most realistic ones--those drafted by actual space agencies--typically feature a collection of prefabricated surface modules constructed on Earth and then placed at the target site, where they get connected by tubes or something. These bases are designed so that a small team of scientists can perform science tasks. Long-term habitation, colonization, or similar goals are simply not (yet?) in the scope of space agencies, and digging tunnels is a heavy industrial process that's nowhere near the critical path of "doing science".

All that said, if you take a look at the more "serious" proposals for Mars or Moon colonization, extensive use of sub-surface space is often made. For example, many of the proposals generated by the semi-frequent Mars Society (like the cities one) incorporate extensive use of tunnels or other underground structure.

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The answer by @fraxinus makes some very valid points, particularly about having prior knowledge of the rock in which the tunnel would be established. This would require a campaign of drilling core samples along the proposed route. Geomechanical experts would be required to assess all such data & recommend ground reinforcement/support requirements & possible changes in alignment. Would there be a seat for such a person on a mission?

Not relevant for the Moon, because it didn't have an weathering environment, but it possibly might be relevant for Mars: knowing where the base of oxidation would be for the given project would affect either the depth at which the tunnel was placed or the ground support required for it.

Weathered rock is rock that has deteriorated due to the prolonged effect of weather: oxidation, effects of water, heat stress, etc. The base of oxidation marks the bottom of the weathered zone. Beneath that will be a transition zone before solid unweathered rock is reached. Depending on the location on Earth, some desert locations can have weathered zones down to 60 m below the surface. Having a weathered zone down to 120 m is not unheard of. Weathered rock usually required more ground support than unweathered rock. This would add additional expense to the mission.

Another thing about using tunnel boring machines is they can easily become stuck. Will the off Earth personnel have the expertise and resources to free a stuck tunnel borer? What happens if they don't? Who ultimately ends up paying for the disruption, or possibly cancellation of the tunnel? What would happen if the tunnel were to be a critical piece of infrastructure?

There are many unknowns associated with tunneling off Earth. Constructing a 3D printed habitat would be easier, less problematic, have fewer unknowns and be less expensive than a subterranean tunnel.

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  • $\begingroup$ Oxidation is (still) not a thing on Mars. $\endgroup$
    – fraxinus
    Apr 15 at 14:40
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    $\begingroup$ @fraxinus: Or is it? Don't forget oxidation by water. Oxidation may not be a significant event now, but Mars is red. It's covered in oxidized iron - rust. $\endgroup$
    – Fred
    Apr 15 at 21:15

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