In my world, there is a race of fish people and merfolk. For a brief description: They are a mix of the Creature from the Black Lagoon and Luca. For pictures and extra description: What conditions could fish people evolve to be in their current humanoid form?

They have their own underwater kingdoms and civilizations, much like the land-dwellers. However, because of swimming a lot, they do not need as much roads/bridges. For the bigger cities, how would cities and streets be planned for humanoid fish people?

Note: Ironically, the fish people do not settle in the deep ocean. This is due to trading and diplomatic relationships with the land-dwellers, resulting in most cities being built around thousands of feet underwater, lowest major city being at around 1,000 meters. Other considerations include:

  • Animals such as whales being used to carry around cargo
  • Guiding schools of fish
  • Their own aquariums (though their more like zoos to them)
  • Eventually as technology progresses, they eventually use submarines like trains or cars
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    $\begingroup$ Interesting question, but we really need much more information. What is their social organisation? Do they have privacy taboos? How do they reproduce and how are young raised? Are they vegetarian, carnivorous or omnivorous? Do they preserve food or do they always eat their food fresh? Do they wear clothes - if so, what? When they sleep, do they keep swimming on autopilot, or anchor themselves to something on the sea floor, or what? Lacking that information we can't even guess at what a dwelling might be like, let alone try to design an entire city. $\endgroup$ Feb 25, 2023 at 6:22
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    $\begingroup$ 1000m pitch black and very little oxygen... why build a city there? $\endgroup$
    – Jasen
    Feb 25, 2023 at 6:46
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    $\begingroup$ What ocean currents are there? You do not need bridges, but it may be hard to swim against the current. There may be enclosed roads so they can travel in all weathers. Or are they in the abyssal ocean, which is pretty still. $\endgroup$ Feb 25, 2023 at 13:54
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    $\begingroup$ 'do not settle deep' yet mentions 1000 m .... You have a strange definition of of not deep. By not deep I would have expected 10 to 100 m in depth. They really would want/need to be in zone where light penetrates. $\endgroup$ Feb 27, 2023 at 20:40
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    $\begingroup$ @GaultDrakkor Sorry, what I meant was that most of the cities are not built too deep, but with a few exceptions being 1000 m. $\endgroup$
    – Crafter
    Feb 27, 2023 at 20:53

4 Answers 4


This is a very interesting scenario, there are so many things to consider:

Infrastructure There is hardly any need for streets, bridges etc. Everything and everyone is able to "fly" (swimming but in 3D) around as they wish. @TheDemonLord mentioned ocean currents but this does not seem very efficient since the would have to be created artificially to be really useful and then it is just easier to propel humans and submarines directly instead.

Structures The need for structures is also much lower. There is no weather to protect from and the temperature is always the same (4°C) which is the preferential temperature for your humans anyway since they evolved in it. So the only reason for structurs is maybe the need for privacy and protection from noise. Basically all public events could just take place outside. With the right amount of air containers, structures are effectively weightless so they are a lot less massive than above-water structures since they do not have to support their own weight. They would probably look more like zeppelins anchored to the ground (or not anchored, it is not that difficult to move them around). The dimensions of the bouyancy creating upper part and the lower living part would be inverted (big living part, small bouyancy part). Pressure is not a problem, since the inside pressure is the same as outside.

Traffic Everything will take place in 3D including traffic. The city will be 3-dimensional, going vertically is basically the same as going horizontally, so traffic (swimming humans, later submarines) would probably just move all over the place in the beginning and will have to be restriced to lanes later when speed and density increases. This might look like this:

enter image description here

(Except for the buildings)

Moving around heavy objects will be much easier since everything can be weightless with the right amount of bouyancy created by air tanks. The term submarine for your vehicles might be misleading, they do not need to be massive at all. A lot of the personal transport would probably take place on scooter-like vehicles since there is no weather and hence nothing to protect from except the driving "wind".

Sewage systemy might be challenging. I assume a pressure based system like it is used in space could work. If your looking for more inspiration for details, solutions for living in space might be helpful because a lot of challenges there are similar to your cities.

I cannot stress enough how challenging the 3D part is. All cities we know are essentially 2D regarding most of the planning. Your cities can be entirely 3-dimensional which creates a lot of interesting situations for your planning and stories which may be hard to convey to your readers but very great if it works.

  • $\begingroup$ The 3D aspect definitely adds many complications but also opens it up to more creativity. I have often thought that regulating transportation is one of the hardest things. Though there is plenty of media on this idea for flying cars, the issues that arise (think an accident occurring, intersections) lead to many complications that are not often addressed $\endgroup$ Jun 1, 2023 at 14:56

Interesting question - my immediate thought is Underwater Currents. These would form the equivalent of Highways in a modern city. Sure, everyone swims, but to move anything in bulk would absolutely use the Underwater Currents.

Your major city infrastructure would be built around these - things like your mining/aquaculture/primary industries would be arranged around these currents to move cargo/items.

In addition, with the building, these underwater currents would have structures built up around them to guide/smooth/define the natural flow to make it more laminar and predictable.

You would also see a different kind of Verticality to buildings - Tall buildings like Skyscrapers may or may not be possible underwater (the hydraulic pressure on a tall, flat-sided building makes me think they wouldn't be stable) - but because of the buoyancy in water, some of the issues with building 'up' on land is eliminated. So the verticality of building would be drastically different, lots more rounded/blended structures, but could also use tethered buildings (like an Oil Rig, but just below the surface).

The other consideration is Light - Would they need light? would it be a luxury? e.g. a house near the surface is a privilege?


Any City needs to accommodate the requirements of Urban Planning - an ancient yet still currently practiced profession

Urban Planning is an ancient and modern profession - one of the earliest in human history is Vitruvius, who coined a 'book of rules' for ancient Roman cities which almost all modern cities are based on:

  • Health and Amenity: How the city deals with waste products, and creates a healthy environment for inhabitants. In Vitruvius's day (and modern times) this means don't locate your city in marshlands or swamps, locate where fresh air is possible, not where floods occur, and need ways for waste to be extracted (using streams etc.). For your underwater city this translates to: not locating the underwater city in brackish or low oxygen water, not too deep, plenty of water flow but not too much. Locate lavitrines (or waste areas) on the edge of cities, or devise mechanisms to dispose of fish waste adequately.

  • Economics: The main point of cities is to form around an economic activity. The best cities today have a variety of overlapping economic purposes. In Vitruvius, this was the market square (ie. a central place where interaction of inhabitants occurs to allow development and economic function). In your case, the 'market square' could be a 'market cube' or 'market sphere', with all structures radially leading to this hub to allow for interaction in 3D of fish and inhabitants.

  • Governance: All cities need administration procedures not only in order to keep law-and-order, but also to orderly plan its growth, monitor and maintain structures, assist property development etc. Vitruvius used a network of streets arranged in a grid to allow for easy property ownership, with a main cardinal grid with the market square at the centre. Most modern cities are based on grids to allow this easy way finding, and easy property development. In your case the city is semi-3D, so the grid could be a cardinal grid of NS, EW and UD.

  • Development over time: As always, cities evolve and develop over time, lest they become redundant and abandoned. If a city is a mining city, when the mine is exhausted the city is no longer economically viable, and people leave. To this end, most cities need a variety of economic growth activities to keep the population in the city (otherwise they just leave). In your case, think about what the city is producing, what economic activity it specialises in to allow it to exist over other cities. Then think about complimentary industries that allow it to survive - unique to fish (ie. food refining, university or tertiary education, governmental and administrative functions, secondary industries).

You will find that your underwater city is influenced by issues that are common to all cities, only just in a 3-dimensional manner.


From the videos I've watched of sunken ships being explored, I think the materials will need to be smooth/slick surfaces like fired & glazed ceramic. Every surface will quickly be colonized by filter feeding lifeforms (such as anemones and sponges) who need currents to push food passed/through them. Perhaps your people might want to deliberately encourage certain types of sponges/anemones while discouraging others.

I suspect shelter/dwellings will require 2 conflicting goals: free flowing water (to provide oxygen) and being able to seal the place (in order to keep out parasites and things that want to hide from predators). Children of Ruin might give some ideas for octopus-type people.

My suspicion is that most structures will be cylindrical as flat sided structures would be damaged by water currents. Or would need to be extremely robust to survive ocean currents.


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