I'm writing an adventure story set in the medieval age, our hero came(dived) across an underwater city with an estimated population of several thousands at depth of hundreds of meters below sea level.

I'm trying to come up with a convincing technology which could seem plausible at that time, but after many brain storming attempts all I have got is a mental block.

Note: I'm writing sci-fi but not fantasy so avoid magic as much as possible.

What kind of technology could the medieval people use to build an underwater city?

  • 2
    $\begingroup$ Without sufficiently advanced magic, your hero - assuming s/he's human, and not e.g. a mer-person - just can't do that. You can't even do that today without very advanced, professional equipment. Anything over about 30 meters, and you run into problems with nitrogen narcosis and the bends (as gas dissolved in the blood under pressure comes out). A bit deeper, and you get into pressures where oxygen becomes toxic... $\endgroup$
    – jamesqf
    Apr 8, 2016 at 4:49
  • $\begingroup$ What kind of city are you looking for? Is the city filled with water or are you envisioning a sort of dome with air inside? Assuming that it is not filled with water, does it count if it is underground? $\endgroup$
    – Ovi
    Apr 8, 2016 at 5:06
  • $\begingroup$ Drop the hundreds of meters and the puzzle becomes something that can be managed. Quick check puts the air pressure at 10+ bar, ~160PSI. Which is not something humans can deal with. They could discover a city that was built 'by the old ones' and just use it. Building from scratch in the middle ages means there is a reason to do so, mining, fishing, protection. Otherwise the society will build to profitable energy returned vs energy spent. $\endgroup$
    – dcy665
    Sep 8, 2017 at 3:35

6 Answers 6


Quite a lot of the technology for your classic inverted goldfish bowl city doesn't exist quite yet. Amusingly enough, looking at why it might not work might give you ideas in why it would.

Lets start with the basics - oxygen, food, water and light.

You are going to use up oxygen, and deep underwater you may not have the same sort of photosynthesis needed to reprocess oxygen, and many plants would not grow without sufficient light. You might have to handwave a bit for that and claim there's some sort of (IIRC) weird primitive slime that or other lifeform that extracts oxygen from some easily found element. You'd also want some way of replenishment of air - a positive pressure would keep water out and you'd likely have losses anyway.

Food - fish I suppose? That would need people diving so maybe airlocks - and you don't quite have the liberty of simply pumping out air since you might have a finite amount of it. Maybe grow mushrooms. If you could grow plants, it would help with sanitation I suppose, helping reprocess waste products. Maybe hunting whales or other sealife nearer to the water.

Water - you're in the sea. There's lots of water right? But most of it is salty. You'd need a source of fresh water water for sanitation, and being under water, some way to pump out waste products without pumping it in. You need to pump water out. And no culture's had much luck building entirely underwater.

Light - photosynthetic algae would be a source, as would maybe natural hydrocarbons. Whale oil or hydrocarbons would be handy, possibly.

The next thing to look at is essential technology/biology to solve these problems.

Constructing a city underwater seems implausible, but would it be needed? I'm thinking a medieval version of rapture from the first two Bioshock games - a series of tall structures linked up by tunnels. What if the buildings were there first?

We'd start with a city of large solid structures - maybe of stone. As time went on, the city started going underwater. Rather than abandoning the city the local residents shored up walls, plugged gaps with pitch and started planning ahead. Maybe they discovered a unique local aquatic mould that fed off human waste products and kept the air fresh, and found glowing algae growing on the walls.

As the water levels went up, they started building, first retaining walls, then closed them up to form tunnels between buildings, as well as raised up buildings, forming a linked complex of semi submerged buildings. They also discovered the rudiments of various pump designs. However they could not keep doing this for ever - so you'd need an equivalent of Da Vinci to invent pumps and an airlock system. However as the water got deeper, the society got more insular.

Freediving seems a useful skill, but assuming a truly underwater city, they would be limited to 200-300 meters with world class freedivers. A series of diving bells supplied by the same handwaving processes might extend that somewhat, possibly with small settlements ringing the primary complex.

Part of me thinks building and sinking buildings would be amusing too, but I have no idea who would be that crazy.


It might work a bit better if you can separate a city that can be accessed by diving that far underwater from a city built that far down underwater. If you had a city built deep underground, that somehow managed to tap into a network of deep caverns with water access (without compromising them), you might end up with a city that builds into and uses those caverns, and ends up with a stable access point where your diving hero can find it. It will also be a lot more plausible if you can accept a shallower city, after all finding the underwater access point to an amphibious city (built partly underwater) can be pretty striking even if it's not quite so far down.

I recall a story where a city was built into a dry portion of a cenote, a cave system with a deep underground water access. Usually the cenote will be filled to the ground water level, so not particularly useful for dry caves deeply underwater (especially not with surface access, which your city would also need to survive), but if you assume a tricky geology and a number of features your city happens to be able to take advantage of (rather than deliberately causing, which they wouldn't have the technology to do yet), it might not be quite so impossible.

So, maybe you start with a deep cavern system filled with water. It needs to have deep ocean access - maybe the system is through a cliff next to the sea (which drops all the way down to give an opening at the required depth). Or perhaps it is an island over an old, inactive ocean volcano, where some of the lava channels are waterproof stay dry (and some have access to the island top and therefore air), even though most of the mountain is under water, and there are systems with access to water. Freshwater might occur in caverns without direct access to the sea by filtering in through the ground rock (aided by pressure and osmosis) into parts of the cavern system where there isn't a direct connection to the sea. In other places, sea-water would have direct access to the cavern system, and maybe there would be some brackish water tunnels where they systems interconnect.

So, parts of the system get cut off from each other and the ultimate water source (caverns that hold water, should be water proof enough to keep water out if that initial access is blocked) perhaps through earthquakes. There ends up being dry caverns extending deeply under sea-level, in addition to the fresh, sea, and brackish water systems already imagined - although there should be access somehow between the systems which doesn't cause them to equalize, maybe underwater tunnels separating air systems and rooms with air pockets separating water systems, working as crude airlocks. They can even be made on purpose, once a few natural ones let your people figure out how they're keeping they systems separate - and why that's necessary.

And you have a people who move into the upper levels of the cavern system, perhaps to escape some outside threat, or maybe just for better access to the filtered freshwater and the cavern system. Maybe they needed to live underground because they needed all arable surface land (if on an island, for example) to feed people. They might spend generations slowly building themselves deeper into the cavern system - and if they're using freshwater in a more isolated system faster than it seeps in, they might excavate their way downward, using newly drained caves as extra storage and eventually living space, and following the water source to keep moving down. Of course, they can use their technology to make the caverns more habitable, and try to keep the various systems separate but accessible, as they build their city downwards.

The crude bellows and pumps of that age (used for airflow in forges and such) might be not be sufficient to keep the water out of a true underwater city, but they might be enough to keep airflow and oxygen circulating to a livable degree in a deep pressure cavern system with access to the open air at the surface. They understood enough about airflow to to feed the fires with good air in their forges, this is just a bit of a larger scale application to feed good air to their people. Maybe they can supplement their oxygen with cultivated pools of certain types of algae, in places where light might shine for photosynthesis, but perhaps the air is more stagnant - or even using bits of polished metal to reflect light in for just that purpose. They would not be racing race down to build the city hundreds of meters below sea level, they would be slowly, over generations, edging their way downwards. There would be a lot of slow experimentation, and these tricks would let them slowly make marginal areas more habitable, and as they adapted to the area they would find more tricks to make the place habitable, and more tolerance to marginal areas.

Also, as they descend I expect they would be living under more and more pressure - since big pressure differences would cause the cavern system to flood or collapse. But, they might be able to adapt to that too, if the best divers can visit 200-300 meters it shouldn't be impossible for a population to adapt to 100 meters below sea level, just like populations can over time adapt to living at high altitudes. The worst problems seem to some from moving between different conditions quickly, but long term living is easier to adapt - though the best divers from this population might be able to go a bit deeper than from a population adapted to surface pressures. Maybe the population would split into interconnected settlements at different levels, who had problems going too far outside the level they were born in without great care. This might mean after a while it is easier and possibly safer for the divers living at a deeper level to navigate the cavern system to access the ocean (for seaweed, algae, fish, or any other resources) from the deep underwater access point your hero found, rather than ascend through the city and access the water from the surface.

  • $\begingroup$ I think this answer should be the one marked as correct. $\endgroup$ Aug 3, 2016 at 1:27
  • $\begingroup$ @Renan - thanks, I'm glad you liked it. $\endgroup$
    – Megha
    Aug 3, 2016 at 2:00

Impossible. There are multiple problems here:

1) You can't free dive to hundreds of meters beneath the surface. While you just possibly might be able to create a diving bell (probably not, the compressor needs to be pretty powerful and I don't think they could do it) they're not going to be able to create the mixed gas mixture needed.

2) The same pressure & air problem exists for the city. Even at the smallest depth that satisfies "hundreds of meters" you're already way into the realm where special atmospheres are mandatory. You have no way to make the atmosphere and you're going to have a very hard time keeping the pressure out to do it with a normal atmosphere in there.

3) It's going to be almost pitch black at that depth. What's their light source? I'm not aware of anything that doesn't need oxygen or electricity--you're going to have to be supplying a lot of oxygen to light your city. (Before someone says "bioluminescence" the creatures that produce it use oxygen.)


Some sort of bloodsucking remora like symbiote that trades food for intravenous oxygen, perhaps it evolved that way to prevent the host from being singled out as easy prey by predators, maybe they've been selectively bred to keep the host hydrated too.

The symbiote would have to be quite large to provide enough oxygen to keep both it and its host alive which means it will require a considerable amount of food, whether its being fed by the host or feeding upon the host directly this society is going to need access to lots of calorie rich food.


I like "Lightness Races in Orbit"'s idea of the pre-existing surface city adapting to underwater life.

Fresh water: I haven't been able to find the source, but I recall an ancient Greek city (Syracause? Athens?) used a submarine spring as one source of fresh water, stimulating some early technological development.

Air: as with the fresh water, a large pocket of air could be trapped, like natural gas or an artesian aquifier, in a permeable rock (sandstone?) beneath a layer of impermeable rock formation (limestone?). This works perfectly if the local geological region has undergone considerable subsidence.

The problem for your city is that (a) the air supply is non-renewable (b) they may not understand that, and (c) at a depth of hundreds of metres, nitrogen narcosis is going to be a huge problem. The only way I can see around (c) is that is that there ARE underground reservoirs of helium, usually associated with radioactive ores that have emitted the helium. If the underground air is mixed (intentionally or otherwise) with helium, you will get a https://en.wikipedia.org/wiki/Heliox or more accurately https://en.wikipedia.org/wiki/Trimix. The reduced % of oxygen isn't a problem if the city is operating at the pressure of the surrounding water (dozens of atmospheres). Given medieval materials, they probably don't have any option but to keep it at ambient (high) pressure.

If the helium supply is sabotaged/runs out, your city inhabitants are going to quickly become dreamily intoxicated due to nitrogen narcosis.

Another problem would be decompression to reach the surface - days of gradual ascent would be required. A dedicated "ascent tower" could be an option, water filled but with bubbles rising to replenish the divers clay/leather/glass jar "helmet".


Since you are writing a sci-fi the only technology I can think of is : the bubble maker.

Each citizen in your city lives inside a personal bubble, they can eat, work, sleep and do everything normally without breaking it. Each bubble has a 24 hours supply of oxygen after that, every citizen must go to this huge medieval machine to change his personal bubble.

  • $\begingroup$ And just how, with medieval technology and without magic, would that work? $\endgroup$
    – Monty Wild
    May 20, 2016 at 2:40
  • $\begingroup$ @MontyWild Sorry I forgot about the no magic rule, that's why I was down-voted. But still you can make bubbles underwater even with medieval technology : physics are the same. $\endgroup$
    – Javert
    May 20, 2016 at 13:41

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