# How to keep a city floating beneath the surface of an exoplanet's ocean in place?

How would I be able to explain a city that is underwater, floating in place and such. Without it constantly moving. Basically, how can I explain how it stays in place and doesn't get pushed all across the ocean from any current that passes by? That is really the main issue, currents causing it to move over time, essentially being more like a glorified submarine than a settlement.

The city is deep underwater, around 400 meters at the shallow end, and is basically a bunch of passages connecting large hubs. I want it to stay in place, but I just can't think up a good reason. I don't have values for the exact size but it is definilty on the large side and is rather spread out.

I already have an explanation for the reason why it is able to float in place, just make bouyant force equal to the weight. Which I can try to measure out and try to make work, it isn't too hard to calculate out the required volume according to the mass.

It cannot reasonably attach itself to the bottom of the ocean, as it is way too deep to feasibly reach (over 100 km, it is on an exoplanet, not Earth).

Which is just leaving me lost at how I can explain it. And it is leaving me anxious as I really like the idea, but I also don't want to completely ditch science and just say "oh, they use a magic unobtanium metal that stays in place" or something like that. So please do not give things like that as an explanation, I am not looking for that.

• Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. Sep 26, 2023 at 0:44
• Why is it absolutely vital to stay in place? With 100km deep oceans, easy access to any underwater ground seems unfeasible. Why then not float around on the currents? It is also big enough that currents should flow quicker than the city, allowing resources to reach it. Are there still landmasses it can hit? Wouldn't it just anchor at an underwater mountain? I think these additions can help with an answer. Sep 27, 2023 at 20:49

You cannot use hydrodynamics, currents etc. alone to justify the floating city staying locked in place. The forces involved are too large and any equilibrium would probably be unstable.

You might have a city that floats in the sea, above a large and denser "sea" made of saltier, colder water. That configuration would be stable, with the city at the bottom of a thermohaline vortex which could, why not?, be stable. Global warming permitting.

Otherwise, if you can engineer, through a judiciuos use of flotation, hydrodynamics etc., a city that is almost stable - then you can lock it into place using flux pinning. You need a very large city with a superconducting core, with a large cross-section with respect to Earth's magnetic field (the reaction available depends on the magnetic field multiplied by the cross section: since Earth's field is very weak, you need a proportionally larger section).

Of course you need a large quantity of room-temperature superconductor. But once you have that, you can flux-pin your city.

• While this seems very interesting, I don't think it is remotely feasible to do this at a scale larger than a few centimetres, I believe I already found a solution with just, trying to use propulsion to move it back when it moves out of place. Also, what you quoted wasn't my question, but the editor's. They decided to randomly add their own question with no indication that it was their own. Sep 25, 2023 at 18:03
• @DanceroftheStars sorry; I had not noticed this edit and it brought me off track. As I understood the question, the purpose was that of not employing any visible means of propulsion. Of course yes, attitude-adjusting propellers would be the simplest and most efficient way to do this. Sep 26, 2023 at 6:55
• @DanceroftheStars Except that you cannot just use propulsion to move it back. If would take an insane amount of power to move a city as it's mass is enourmous. It is more immersion-breaking to move the city rather for it to be fixed in place for no reason. Sep 28, 2023 at 11:07

# You are generous with what "in place" means

An ocean gyre is a set of circumstances that causes ocean currents to move in a circular manner.

The center of such gyres are usually calm, and keeps floating objects and structures in the gyre relatively stable and stationary inside the gyre.

Two real world examples of this are the Sargasso Sea, and the great garbage patches.

Hence, you do not need to anchor your floating city. Instead, just place it in a relatively tight ocean gyre, and it will remain within the gyre. No, it will not be perfectly stationary and, no, it will not remain in the same orientation. However, it will not migrate away over the oceans.

• Neal Stephenson did it with his Floating City around the Enterprise in Snow Crash
– Pica
Sep 28, 2023 at 9:17

Lateral Thrusters.

Assuming you have a) adequate and reliable power sources; b) sophisticated inertial navigation system (and/or a combination of several navigation systems like GPS): and possibly c) a sonar system mapping your city's position relative to (large) markers dropped onto the sea floor

You can use a grid of thrusters as per the wiki link here. [1]: https://en.wikipedia.org/wiki/Underwater_thruster

Large ships like ocean liners use such thrusters to assist with docking maneuvers. In your case a network of thrusters is attached along the outer perimeter of the city covering 360 degrees. They are electric powered and in a computer controlled 'always on' mode. A central control room somewhere monitors the status of all thrusters in the network 24/7 and makes sure the computer is keeping the city in the desired position. Warning: like it or not even with the best control systems possible given the forces at play and the scale of the problem the city will always 'out of position' to some small degree so 'in place' will be a relative term, more of a zone with tight parameters than fixed point.

Also FRAME CHALLENGE: While 100 kilometers or so is deep multiple large and strong composite material cables may be able to support both their own weight and the strain of the holding the city in place. But you'd need someone to do the math.

• 100 km long cables holding the city in place is more plausable than using engines to move the city. It would require insane amount of fuel, if it would be possible at all. Sep 28, 2023 at 11:11
• I feel it would be possible from what I researched, the pillar that is, the main issue is that it would be absurdly expensive. But I can just try to use it, and lampshade how expensive it was. And find some internal structure which could handle the lateral forces. Sep 28, 2023 at 21:10
• @Negdo For a start the thrusters wouldn't necessarily need an 'insane amount of fuel' they can be powered electrically which would actually be simpler to engineer. But even if they were the city has a whole ocean at it's disposal either as a direct form of fuel (hydrogen) or as a location for growing microorganisms in 'tank farms' that can be converted and refined into fuel.
– Mon
Sep 29, 2023 at 4:21
• @Mon Simple physics disagree with you. Estimation of the mass of a city is in range of 10^13 kg. Cities are massive and huge. As you know, F = m*a, F being force, m mass and a acceleration. Even for small accelerations (and you would need better than 0.000001 m/s^2, since you have to counter the influence of currents) the forces needed are insane. Largest ship has a mass of 6*10^9 kg and it's engine has around 80 MW of power. This city would need a few orders of magnitude more, since it weights that much more. A generous calculation would make that 100 GW just for thrusters... Sep 29, 2023 at 7:14
• @Negdo. All true but that level of power needed to provide the main propulsive force for large vessels. The city doesn't need to push through the ocean at the same speeds a large tanker (or in this case submarine) does i.e. 18-30 knots. Nor would all the thrusters be engaged all the time. I would also assume that the power available scales with the size of the city. If they truly are reliant on thrusters for station keeping the engineers would have to scale the power available accordingly. And yes, thrusters are a poor second choice next to sea floor cabling but that option wasn't allowed.
– Mon
Sep 30, 2023 at 4:35

Two currents pass each other, one near the surface, one deeper beneath the surface, the city performs a constant ladder jump beneath the depths, thus roughly staying in the same place.

Here are some of my ideas for how you underwater exoplanet city would work, whilst also not being tethered to the ocean floor.

1. Tethering to Underwater Geological Formations:

• The city could be anchored to large, stable underwater geological formations like mountains, cliffs, or deep-sea ridges using strong, flexible tethers. These tethers would allow for limited movement and flexibility while still keeping the city relatively stable.
2. Hydrodynamic Design and Countermeasures:

• The city's architecture and design could be hydrodynamically optimized to minimize resistance to currents. Additionally, the city could have built-in mechanisms like adjustable fins, sails, or ballast systems that allow it to counteract and navigate ocean currents effectively.
3. Dynamic Buoyancy Control:

• The city could have a sophisticated buoyancy control system that allows it to adjust its buoyancy in response to changing currents. By modulating its buoyancy, the city could maintain its position or move strategically within a certain range.

In the end, we would still need to know the size of the city and how it’s engineered to build it a proper support structure/system. I hope this answer halped!

It cannot reasonably attach itself to the bottom of the ocean, as it is way too deep to feasibly reach (over 100 km, it is on an exoplanet, not Earth).

Anchor it to surface ice, then. From below the ice.

Free floating ice won't do, but if you've got an ice sheet connected to permafrost that never breaks away, you can attach to that.

Or attach laterally to islands via very long cables. You need at least two islands, but the more the merrier. If you have the technology to build a city underwater and on an exoplanet, this should be easier than building a space elevator.

Or, better yet: attach to submarine internet cables. This could even be the reason why the city even exists. Somebody needs to run maintenance on those cables. Maybe the city is even able to run over the cables as of they were tracks to move from one point to another as required, but mostly it would remain in place.