One of the points made in a number of recent questions regarding underwater races has prompted me to review one of my older pieces of world-building. Namely: the difficulty of producing common structural materials like steel or concrete in an aquatic environment.

I have a species of intelligent, fishlike aliens that live on an ocean world - a world much like ours, only the sea level is high enough to submerge all but the highest peaks.

This species is the product of a prolonged uplifting program by a much more advanced civilisation - their technology is by now distantly descended from a wide variety of prefabricated factories introduced to them during the initial uplift. Accordingly the bootstrap problem isn't a big deal, since it can be assumed they acquired any necessary knowledge and equipment from their erstwhile benefactors. Similarly necessary raw materials such as ores can be safely assumed to be present.

Technologically they're what we might think of as a contemporary - they're comfortably industrialised and have turned their attentions to spaceships. What components they can't fabricate underwater are produced in aerated industrial complexes built in shallow parts of the world, supported by vast logistical networks. Suitable locations are limited, though, and I'd prefer not to rely on shallow plateaus for all industrial production. In general for this species, air-breathing manufacturing is expensive and inefficient but nevertheless possible - their motivation for moving industry to aquatic manufacture is a matter of efficiency more than for lack of alternatives.

What I'm looking for is a structural material that can be produced underwater with little to no reliance on atmospheric gases or access to the surface. Optimally it would be usable in ways comparable to steel, but it should at least be suitable for construction. Typical applications would be shelters/homes (against wildlife and aquatic weather), storage, workplaces, and so on.

  • $\begingroup$ Is access to ores part of the question, or can we assume they just have it? $\endgroup$ – Mołot Apr 18 '18 at 9:55
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    $\begingroup$ I don't think this question can be answered without knowing the basis of this civilization's "airless" industry -i.e. What can they do now, and how? So much depends on access to air and fire. $\endgroup$ – Spencer Apr 18 '18 at 10:07
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    $\begingroup$ What kind of structure do you want to build and where? Is it e.g. fixed to the ground? I don't think it is self-explanatory at all what kind of structures an underwater civilization would even have the need for. Once this is established, people can think about what kind of materials you can build them from underwater $\endgroup$ – Raditz_35 Apr 18 '18 at 10:13
  • $\begingroup$ @Spencer At this stage of world-building they can get anything they need with air-breathing industry (ie. waterproof near-surface factories with air intakes) - for them developing airless industry is a matter of efficiency more than necessity. In theory they could just produce all their materials near the surface. With that said the research that prompted this question suggested that 'airless manufacture of structural materials in an underwater environment' was a fairly significant obstacle to begin with $\endgroup$ – Pingcode Apr 18 '18 at 11:05
  • $\begingroup$ @Raditz_35 Their needs I would say are comparable to our own, albeit not identical - shelter is a necessity for many species of sealife (both against predators and against underwater weather), and we can readily suggest the need for places to store things, places to work, and so on $\endgroup$ – Pingcode Apr 18 '18 at 11:28

15 Answers 15


The Direct Reduced Iron (DRI) process makes raw iron with inputs only of electricity and natural gas. (source)

More info on Wikipedia.

If you have access to natural gas, iron ore and electricity, and enough thermal insulation to keep 1200°C, then you can have steel. If you have access to chromium and other alloying elements, you can have most of the kinds of steel we have.

You will have problems with rust, and you will have problems to prevent water flushing in thorough your CO2 exhaust, but these can be solved. You don't need "comparable to steel", you can simply have steel.

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    $\begingroup$ interesting, but can one access electricity without having metals? $\endgroup$ – L.Dutch - Reinstate Monica Apr 18 '18 at 10:55
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    $\begingroup$ @L.Dutch see the question. Especially: "Accordingly the bootstrap problem isn't a big deal, since it can be assumed they acquired any necessary knowledge and equipment from their erstwhile patrons." so yes, they can. They just got their first metals as a gift and now they can make more. $\endgroup$ – Mołot Apr 18 '18 at 11:05
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    $\begingroup$ @L.Dutch You could use a metal that is found in suitably pure forms in nature - e.g. Gold, which is also easy to shape - and a lump of loadstone to craft a crude dynamo. $\endgroup$ – Chronocidal Apr 18 '18 at 14:36
  • $\begingroup$ Since the question states What I'm looking for is a structural material that can be produced underwater with little to no reliance on atmospheric gases or access to the surface. can this be produced underwater? It seems like getting it up to 1200C would cause a bit of a problem. It could be a small pressure sealed vessel where the steel is forged I suppose, that's a lot more trouble than making it on one of the shallow places that has access to the surface. $\endgroup$ – AndyD273 Apr 18 '18 at 16:33
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    $\begingroup$ @BobTheAverage Neither is copper. Copper coils seem to work. All you need is an electrical conductor, not a static attraction. Ferrous materials are commonly used to shape/concentrate the magnetic field to where it does the most good, but they are by no means required. $\endgroup$ – AaronD Apr 19 '18 at 2:47

Domesticated Molluscs

enter image description here

Giant clams are just one sort of shellfish, but there are potentially many others of various shapes and sizes. You don't even need to grow clams this big so long as you are content using lots of little shells in your building. In particular, I'm thinking of extinct Rudist bivalves that came in a dizzying array of shapes and sizes and were the major reef-building organisms of the Jurassic and Cretaceous. Why smelt or mine or refine, when you can simply grow!

Domestication and selective breeding causes all sorts of major changes to animals. Wild aurochs didn't produce five tons of milk per year; and wolves look nothing like pugs. In this case, the easiest way to make the appropriate building materials is to grow the clam in a box of some sort. The clam's shell will naturally be fitted to whatever shape it is grown in. This gives all sorts of options for growing building materials for specific purposes.

I'm thinking that you would encourage the clams to grow into shapes that lock together. Gravity pulling a building down isn't so much of a big deal underwater, since there is a significant buoyancy effect. But currents push with much more force than wind. You would assemble a structure like a puzzle with 3-d interlocking clam pieces keeping the structure stable against the tides. Once you build the structure, you cover it with barnacles or corals or something that will help seal everything together.

Sure it would take a long time, but a shell structure glued together with a living, regenerating barnacle armor would be quite the formidable underwater fortress.

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    $\begingroup$ this idea has been approved by octopus everywhere. theguardian.com/environment/shortcuts/2017/sep/18/… $\endgroup$ – Reed Apr 18 '18 at 15:37
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    $\begingroup$ "Nice barnacle armor, King!" "I'm not wearing barnacle armor." "Woops." $\endgroup$ – Willk Apr 18 '18 at 15:55
  • $\begingroup$ Shellfish don't leave empty space inside their shells. It is entirely possible to kill the shellfish and move it out, but at that point it is no longer living and regenerating. $\endgroup$ – BobTheAverage Apr 18 '18 at 18:06
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    $\begingroup$ "Sure it take a long time..." Mightn't the growth time be one factor improved by selective breeding? $\endgroup$ – jpmc26 Apr 18 '18 at 20:46
  • $\begingroup$ For in-world history, the practice of breeding/grooming mollusk shells for construction could have evolved from using all-natural shells plus mud as a sort of brick and mortar combo. $\endgroup$ – DanDoubleL Apr 19 '18 at 21:44


enter image description here

Basically find an underwater volcano and tap it, releasing pressure so it doesn't explode, and direct the magma into block forms. Once it has cooled you can pull the new block out and use it for your building material.

You don't have to worry about cutting and shaping stone, it will fill whatever mold you put it in. You don't have to worry about underwater furnaces or forges to get it pliable, the hard work is already done for you by the planet.

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    $\begingroup$ This is basically a way of making custom sized basalt blocks.. $\endgroup$ – kingledion Apr 18 '18 at 22:05

Coral stone.


coral stone

Quarried coral blocks make fine building materials. Around the world one can find buildings and ornaments made of coral stone and it is quarried and used to this day; linked images are from a company in Barabados.

There is plenty of coral stone underwater. There is no reason your merfolk could not quarry and build with it. Of course they could quarry and build with limestone or granite or any other building stone too, but that stuff seems less ocean-themed than coral stone.

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    $\begingroup$ What would they use to cut the rocks? $\endgroup$ – Simba Apr 18 '18 at 16:08
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    $\begingroup$ @Simba, saws, drills and wedges; much as they do up top. $\endgroup$ – ShadoCat Apr 18 '18 at 18:18
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    $\begingroup$ @ShadoCat How are you going to make a saw without metal? $\endgroup$ – kingledion Apr 18 '18 at 22:04
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    $\begingroup$ @Kingledion - how did the Mayans cut the blocks for Chichen Itza? You can cut stone with harder stone. Coral stone would be great for working with stone tools. $\endgroup$ – Willk Apr 18 '18 at 22:11
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    $\begingroup$ Bio-engineered (or at least domesticated) coral could be farmed to grow in useful shapes, as well. $\endgroup$ – arp Apr 19 '18 at 7:43

Of the structural materials we have on dry-land, just one is maybe available to your aliens: stone.

  • Wood: since water takes care of sustaining plants, no hard wood is needed. So no luck here.
  • Metals: ores might be available, but smelting them requires wood or coal and their combustion. Not a thing for underwater.
  • Concrete: preparing it requires combustion and absence of water. Again, not suited for underwater.

And then you are left with stone: remove the mud layer from the bottom, and cave blocks of stones. Use them as bricks, and you are helped by water in reducing the effort of lifting them. You can even use pumice as floating medium to lift loads.

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    $\begingroup$ Smelting metal does not require combustion. It only requires substance to reduce ore to metal, and source of heat. We used coal / coke for both purposes because it was easy, that's all. For concrete, any source for your claims? I can't find either way. I found that preparing cement requires high temperatures and lack of water during the process (but that can be achieved in CO2 filled kiln) but I wasn't able to find if, and what is required from air. $\endgroup$ – Mołot Apr 18 '18 at 12:41
  • $\begingroup$ @Mołot, error on my side for cement. Air (CO2) is not needed for cement, but for lime. $\endgroup$ – L.Dutch - Reinstate Monica Apr 18 '18 at 13:11
  • $\begingroup$ @Mołot how do you make cement underwater, when what it does is instantly start to chemically react and set when wet? $\endgroup$ – RonJohn Apr 18 '18 at 13:46
  • $\begingroup$ @RonJohn where did you get idea I'm saying it's possible? Have you not read the part about CO2 in my comment? $\endgroup$ – Mołot Apr 18 '18 at 14:00
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    $\begingroup$ Cement will harden and cure under water. Cement hardens due to a chemical reaction with water. I used to build load-bearing concrete in water. I'm so glad I don't have that job anymore! $\endgroup$ – gwally Apr 18 '18 at 18:40

Genetic engineering

If this species is the result of an extended uplift program by a more advanced civilization, then that advanced civilization probably has some serious biotech capabilities - after all, uplift programs don't just grow on trees unless you genetically engineer a Fruit of Knowledge.

And in terms of biological engineering, an aquatic medium provides a lot of benefits - it's a lot easier to add building blocks to a liquid than it is to add them to air.

As such, assuming that an aquatic race will use modern Earth methods is kinda silly. They could work twice as hard to get things done in air exactly the way we do, or they can use some near-singularity user friendly biotech.

For instance, to smelt metals they could inoculate a smelting chamber with a specific set of microbes their patrons left behind and run hot water full of mine sediment through it; the microbes then separate out all the interesting minerals, ready for a purification step (which probably involves other, more specialized species).

To build structures they can whip up a minimal scaffolding made from an engineered seaweed, which is impregnated with certain biomarkers. They then blow calcium and nutrient rich water over the whole thing; hyper-corals attach to the biomarkers, and fill the gaps in with a biological concrete. The stalks of seaweed become the interior plumbing necessary for civilized life, carrying power / data / water.

Unless this species was specifically uplifted by a bunch of short-sighted land dwellers with a one-size-fits-all program, they'd work with their aquatic environment - not against it.


Roman concrete. According to a study of the chemical composition of Roman seawater construction by the Lawrence Berkeley National Laboratory (as reported here):

The Romans made concrete by mixing lime and volcanic rock. For underwater structures, lime and volcanic ash were mixed to form mortar, and this mortar and volcanic tuff were packed into wooden forms. The seawater instantly triggered a hot chemical reaction. The lime was hydrated – incorporating water molecules into its structure – and reacted with the ash to cement the whole mixture together.

  • $\begingroup$ A good idea. They would, however, need to source the materials from dry land and then mix them away from water before bringing them to the sea to actually build something with them. So while it works well as an underwater construction material, it's not going to be easy for a purely marine-based being to work with. $\endgroup$ – Simba Apr 19 '18 at 8:15
  • $\begingroup$ @Simba no - they could simply mix the materials where they needed them. Since they're already wet, no need to add any water. $\endgroup$ – Wayne Werner Apr 19 '18 at 14:10

Navy veteran here, and for a time I worked with ROVs in the offshore oil and gas industry. So here are some of my thoughts on metallurgy:

As someone else pointed out already, there is more than one way to reach smelting temperature. You could drill geothermal wells into the sea-floor. For economic reasons it would probably be near underwater volcanic activity such as island chains (also a handy source of iron-ore from material that's already cooled & hardened). Salt water is basically the most corrosive environment possible for steel to be in though, and I don't think it would be possible to smelt high purity steel alloys in that environment. You'd get lots of pitting and imperfections in the crystalline matrix, and corrosion would start immediately eating away at it leaving voids or delamination, cracks would quickly form at stress points... it would be worthless for precision engineering such as with a spaceship. They'd need to do the smelting in an environment as free of contaminants as possible.

Once its cast and put in the water it will immediately be under chemical attack from corrosion. We get around this by coating our steel with anodized metal plating or specialized paint, I believe at one point the British even took to de-gaussing their hulls which must have somehow disrupted the anode-electrolyte-cathode circuit. Either way, as time goes on whatever you're protecting the steel with gets scraped and flaked away. In subsea engineering they bolt on sacrificial anodes made of a metals that corrode faster than steel does, and replace them every few months as they disintegrate. Since electrical current takes the path of least resistance then salt water completes the circuit from a cathode to the sacrificial anode instead of the steel. Regardless of the coping mechanisms, for this species I think steel would be very expensive to work with and would not be a common building material for their domestic civilization.

Steel for space travel seems unavoidable, especially if they have to essentially build flying aquariums and not just pressurized canisters as we do. For one thing there would be much, much more weight associated with lifting water out of the planet's gravity well. They may even need to settle for suits or even just helmets hooked up to a water supply for the lighter craft.

Hope that helps.


Assuming that physics and chemistry area similar to ours... You could still use air and fire technology under the water.

If they have underwater structures which are, or can be made to be, water tight, then they can pump the water out and have air within.

They could possibly pump air from the surface. Depending on how deep they are determines the viability of that.

Alternatively, they could generate usable gasses to fill the space using deaerators.
They can also split water into oxygen and hydrogen using electrolysis, which gives them fuels for fire based manufacturing processes.

To power the electrolysis you can have underwater nuclear power plants. Nuclear power doesn't require oxygen and underwater nuclear plants would be viable. Underwater nuclear power plants are considered a good idea and a civilisation such as yours could have easily perfected that.


The state of their material science

Your species likely has access to a range of materials of similar versatility to ours, but the processes to create them will differ from ours.

What seem like major challenges to under-water material science probably only problems because we have devoted all our effort to developing processes in air.Their processes will have been designed from the start to assume being under water, and will need air less frequently.

I suspect they would be able to get around the challenge of lack of fire fairly easily - they simply need to mix their fuel with an oxidant, and it will burn under water. One way this could be sourced is by creating oxygen and hydrogen from water with electrolysis.

They are likely to be able to access some air, as an input to their processes, though it would be at a cost.

  • They can be assumed to be capable of migrating to near the water surface fairly easily, as they will need to be in the top 200m of the water column to practice agriculture, which would be needed to sustain their civilization.
  • The buoyancy of waste gases from the processes (or just of water heated with the waste heat) can be used to create a lift to ship air down to the factories.
  • Oxygen and hydrogen can be produced by electrolysis in-situ.

If their manufacturing is on the sea bed pressure will have a major impact. To meet your criteria of sea level "high enough to submerge all but the highest peaks" most of the worlds surface would have to be submerged under more than 1000m of water. The result of this is about 100atm of pressure. This changes the way materials behave, for example gases like Oxygen and Nitrogen are "supercritical" at this pressure and act like something somewhere between a gas and a liquid (assuming temperatures above their above their critical point).

I would suggest that they would be capable of creating/accessing all the same kinds of things as us - plastics, composites, metal alloys, natural fibers, concretes, mined rock etc. However the materials they were capable of creating would differ from what we have in various unpredictable ways - leaving scope for your imagination.

Their priorities

The materials they develop will depend on the priorities they have when they are researching them.

  • The importance of being capable of supporting weight may be less significant, because of the buoyancy created by the water.
  • Resistance to corrosion will be essential, making a lot of our favored metals far less useful.
  • The force of watter currents will be something they have to consider seriously.
  • Insulation may matter less, as temperature variations may be less extreme.
  • They won't have to concern themselves with waterproofing their dwellings, as they will already be full of water.

My conclusions

What's Hot:

  • Composites, such as fiber glass: These are an adaptable class of materials anyway. Fiber glass itself is not good at taking compression, but does have good flexibility and strength - with water taking much of the structure's weight fiber glass may be more useful, even taking some roles filled by steel in our world.
  • Glass, it's resistance to corrosion becomes much more significant under the sea.
  • Natural fiber, seaweeds such as kelp can provide impressively long, strong fibers.

What's not:

  • Metals, many of the metals we find particularly useful degrade far too fast under water.

Silica, Titania, and Chitin

Others have mentioned molluscs and coral. I wanted to contribute hacked radiolaria -- think carbon nanotubes, tunneling nanotubes, and buckyballs, but with silicon. This would be for construction materials, and for synthesis of nano and computing machinery.

From Wikipedia:

  • Silicon nanotube, made of silicon atoms and first reported around the year 2000.

  • Titania nanotubes, created by the conversion of the mineral anatase by hydrothermal synthesis.

  • Chitin, a long-chain polymer of N-acetylglucosamine, is a derivative of glucose. It is a primary component of cell walls in fungi, the exoskeletons of arthropods, such as crustaceans.


Concrete sets due to a chemical reaction with water and therefore concrete will successfully set while underwater as long as it's calm enough to not wash away the sediments in the concrete. http://www.cement.org/cement-concrete-applications/cement-and-concrete-basics-faqs (at the bottom)


Another alternative is to use electrolysis to cause dissolved sediment in seawater to precipitate and adhere to a surface. The process was patented in the late 1970's and has principally been used to repair coral reefs that are damaged. It has been referred to as seacrete, biorock and seament. https://en.wikipedia.org/wiki/Biorock

The downside to this approach is that structures have to be formed upon a metal skeleton that conducts the current used for electrodeposition. And once the current is turned off, the structure is subject to decay. So buildings made of seacrete would require a reliable low power source of electricity or the structure would have to be sealed after construction. The upside is that biorock has been found to have a better compressive strength than concrete. So large buildings and other structures would be very feasible. And since the process evolves hydrogen gas; it is possible to collect a fuel in the process as an additional benefit.


User L.Dutch makes the comment, in their answer, that

since water takes care of sustaining plants, no hard wood is needed. So no luck here.

But this leaves out one several important applications of plant material in building: namely to braid wicker/wattle, to make rope and to weave wood fibre fabric.

In the presence of hard wood, these techniques are often less practical in structural construction. But with no hard wood present, they could become quite important indeed. And while they are usually made sturdy with a backbone or lattice of rigid material, they can themselves be made rigid by layering. For instance, thick ropes can serve as pillars.

To make them even more rigid (even very thick rope will maintain some flexibility), two thick ropes can be bent to form arches, dug into the ground at both ends, and placed such that they cross into the middle, forming a groin vaulted roof:

groin vault

Or they could be placed in parallel, forming a barrel vault:

barrel vault

This works thanks to a combination of stiffness and tension on the ropes.


Interesting Ideas on aquatic civilization development, but there is one point that might make things a little more convenient but more interesting.

If the planet is comparable to ours with oceans deeper then ours; what depth does this Race live at? If they are able to take the lower pressures near the surface of the water and even spend brief periods out of the water without need of pressure suits, then it stands to reason they can use the "ocean" of air quite easily. Can we assume this "Fishy" species swims or is it bound to the ground? If it swims then they should have some sort of bladder system for bouncy elevation, if so the traditional ideas of "up" or "down" don't matter as much as they would to us.

I can imagine "barges" made of large shells and shark skin, and other natural materials (giant fish bones) being platforms for use in industry that require air where these alien invert their perspective going "feet up" and use the "pits" of air "below" to smelt or make glass or even process shells into lime for concrete.

If the idea of a nebulous up or down is too alien they can still have massive structures just at the surface of the water like we have on top of the water today. These would be entire floating buildings or islands. All they need is a bubble from volcanic activity or decomposition to get to the very edge of their atmosphere. Once there everything becomes possible. They could put entire oil refineries bobbing with the critical parts above the ocean and the end products collecting just underneath. This pretty much allow them to produce any material should it be doable in water or above.

Given the technology level, the chic of their world could also have floating buildings with water above the ocean line like a massive flying aquarium. Maybe they have stories of an entire lost city that due to volcanic activity rose above the waves. We use the seas incredibly often so for a comparable tech level society I don't see why they can't use the "Airs" as often or more often.


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