I'm designing an aquatic, fish-like species for a sci-fi setting and I've had trouble deciding on methods of transit, particularly mass transit, that would be viable for them. I've considered a few possibilities:

  • They have personal transit devices that closely resemble 'Underwater Scooters' or diver propulsion vehicles. These would be useful for individual transit, but they'd fill a niche more like a bicycle than a train, car, or monorail.
  • Just using underwater trains. I think this idea has too big of a pitfall - that water is so heavy it would be impractical to make a train full of water, whether it's a regular train, a maglev train, or it goes through a vacuum tube. Would the added weight of water make this much of a difference, or would the added power required still leave underwater trains a sustainable option?
  • Transit infrastructure that is built over the surface of the oceans, reducing the amount of required energy to propel the trains. Probably still not a good idea because it'd be a huge endeavor to make the surface accessible for large amounts of the population, and because the train is still full of water.

I understand it's reasonably possible to make a train move even if it's full of water with sufficient power, but I am looking for something that would come more naturally to this aquatic species rather than brute forcing the sufficient but somewhat difficult possibility of water-filled sea-trains.

EDIT: For clarification, the technology level is (loosely) hard sci-fi although, although I am trying to find an idea that would've been feasible in an industrial or modern era civilization rather than this species of sapient fish people only developing mass transit once they get atomic power.

  • $\begingroup$ why ae boats not the answer? $\endgroup$
    – John
    Jan 4, 2022 at 23:54
  • $\begingroup$ They need to breathe water, I should've been more specific. A boat full of water on the surface of the ocean doesn't really get around the problem that trains, monorails, trams, cars, or submarines full of water would have, at least I don't think it does. $\endgroup$
    – borbytar
    Jan 5, 2022 at 0:05
  • $\begingroup$ What is the technology level of the civilisation? Is the world basically Earth-like, apart from the sapient inhabitants? $\endgroup$ Jan 5, 2022 at 0:24
  • $\begingroup$ The only significant difference between their world and Earth is that their world effectively rotates on its side; otherwise, they're remarkably similar. The species are carbon-based, sapient, and broadly fish-like. The technology level is technically sci-fi/futuristic at present, but I'm looking for something that makes sense for them to develop historically - they could easily make a train full of water with atomic power, but I am looking for a more primitive transit solution to evolve their modern one from. $\endgroup$
    – borbytar
    Jan 5, 2022 at 0:58

2 Answers 2


Before getting into details, let's first look at three assumptions and two concepts that will influence the design of a fish-like species' mechanical transport systems.

Assumption 1: The species can manipulate tools and controls as well as a human, whether this is through appendages, control of a symbiotic species with appendages, telekinesis or whatever.

Assumption 2: The species does not require high-speed flow of water over its gills in order to survive, as long as it is immersed in water it can breathe comfortably at any depth it wishes to travel to.

Assumption 3: The species' density is approximately the same as the water they live in. That is, they do not require weights to avoid floating to the surface and they do not require floatation devices to avoid being stuck on the bottom of the lake / sea / ocean.

Buoyancy: A creature or vehicle with roughly the same average density as the surrounding fluid (see assumption 3) requires no effort to maintain the same depth in the water. One consequence of this is that a train on rails or a "car" on the sea floor can get far less traction than its land-based equivalent and either needs to deliberately be made much denser than the surrounding fluid in order to get traction or needs to have very high friction wheels and tracks/roads. (Maglev is totally unnecessary underwater in terms of reducing friction, although it could be used to propel a train along a track.)

However, it also makes it much easier to make use of three-dimensional space safely. Where there are obvious safety concerns about aircraft flying over residential areas in case pieces fall off or they lose power and crash, a submarine that loses power will just keep drifting along, albeit with possibly significant inertia but in a high-resistance medium (see next point) that will slow it down rapidly.

Water resistance: Water is a much denser medium than air and provides far more resistance to high-speed movement. Forget Aquaman - the kind of high speeds we are accustomed to on land and in the air are infeasible for underwater travel. If high-speed travel is required then the species basically needs to go topside to operate hydrofoils and/or aircraft.

The same density that makes it harder to push through water also makes water currents very powerful compared to wind currents. This means that building any fixed transport infrastructure that can withstand water currents and (close to shore) tidal flow requires much more heavy duty structural engineering than any equivalent land-based structure - feasible for short distance transits (see below) but not for long-range travel.

Early transport: Given all of the above and ignoring any domesticated aquatic species used as beasts of burden, the first feasible "powered" transport system would be a rope tow powered by a turbine located in a powerful current. Think of basically a big windmill built underwater in a reliable current, connected mechanically to a rope loop, maximum length determined by the strength of the rope available. As with those used on ski fields, improved mechanical linkages at each end could allow the addition of the fishy equivalent of a chair or even one or more cable cars. Until the species develops electricity, this will be the limit of what can be done unless they venture onto the land to build steam engines.

Next step - the submarine

Regarding submarines - these are entirely practical for the fish-like species. A human-operated submarine is a pressure vessel requiring a very heavy and strong hull to protect the surface dwellers inside. Importantly, a submarine cruising at a given depth has neutral buoyancy, that is, the average density of the entire submarine including crew areas, hull, engines, ballast etc is close to 1 kg/litre. A submarine for a fish-like species does not need the pressure hull, it needs just enough material to create a streamlined shell - about the density of the typical speedboat hull. In order to achieve the same neutral buoyancy as a human-crewed submarine it will probably require some low-density areas (eg styrofoam filled) to offset the higher density engine, but with the exception of the conning tower, the shape is likely to be the same as a human-crewed vessel, just with far more windows.

The threshold technology for a submarine to be practical is probably the development of electric motors and batteries. I personally consider it less likely that an aquatic species would build internal combustion engines for underwater propulsion before electric motors. Given that no pressure hull is required, there is no reason that a submarine cannot be made as large as required. The flexibility to take any path through the "flight space" of the water without being tied to fixed infrastructure such as cables, rails or roads suggests to me that except for specialised, short distance routes, submarines of varying sizes would be the go-to transport option for an aquatic species underwater at most tech levels.

  • $\begingroup$ Hmm, wouldn't using sail boats (buoys) be easier and faster? $\endgroup$ Feb 24 at 6:31
  • $\begingroup$ @SuperYoshikong easier and faster for what? This is a purely aquatic species, trying to utilise the air currents above the surface of the water would be unlikely to even occur to them and very difficult to implement even if they did. $\endgroup$ Feb 24 at 12:39

Pneumatic tubes except A. much larger and B. full of water not air. The description says, to me, that where humans would walk this species would swim. To speed them on their way an artificial current assist would feel reasonably natural and the speeds reached could be quite high without them having to work any harder than they usually would. You haven't said whether they're solitary or schooling in social habit but very large pusher tubes, with capacities like a super highway, that let them swim through en masse might be preferred to smaller tubes that only let them travel in sidewalk sized groups.


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